[Messaging Clients] README Concept Updates (#18804)

The focus of these changes is to add content for lifetime, thread-safety,
and additional concepts specific to the messaging clients.  The content
and format is intended to match the common blocks used by the HTTP-based
service clients as closely as reasonable.

(see: "Thread safety" and "Additional concepts" here: https://github.com/Azure/azure-sdk-for-net/tree/master/sdk/storage/Azure.Storage.Blobs#thread-safety)

sdk/eventhub/Azure.Messaging.EventHubs.Processor/README.md

@@ -66,6 +66,21 @@ For the event processor client to make use of Azure Storage blobs for checkpoint
 
 For more concepts and deeper discussion, see: [Event Hubs Features](https://docs.microsoft.com/azure/event-hubs/event-hubs-features).
 
+### Client lifetime
+
+The `EventProcessorClient` is safe to cache and use as a singleton for the lifetime of the application, which is best practice when events are being read regularly. The clients are responsible for efficient management of network, CPU, and memory use, working to keep usage low during periods of inactivity.  Calling `StopProcessingAsync` or `StopProcessing` on the processor is required to ensure that network resources and other unmanaged objects are properly cleaned up.
+
+### Thread safety
+
+We guarantee that all client instance methods are thread-safe and independent of each other ([guideline](https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-service-methods-thread-safety)). This ensures that the recommendation of reusing client instances is always safe, even across threads.
+
+### Additional concepts
+
+<!-- CLIENT COMMON BAR -->
+[Client options](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/eventhub/Azure.Messaging.EventHubs.Processor/samples/Sample02_EventProcessorConfiguration.md#event-processor-configuration) | [Event handlers](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/eventhub/Azure.Messaging.EventHubs.Processor/samples/Sample03_EventProcessorHandlers.md) | [Handling failures](#exception-handling) | [Diagnostics](#logging-and-diagnostics) |
+[Mocking](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#mocking)
+<!-- CLIENT COMMON BAR -->
+
 ## Examples
 
 ### Creating an Event Processor client

sdk/eventhub/Azure.Messaging.EventHubs/README.md

@@ -59,6 +59,21 @@ For the Event Hubs client library to interact with an Event Hub, it will need to
 
 For more concepts and deeper discussion, see: [Event Hubs Features](https://docs.microsoft.com/azure/event-hubs/event-hubs-features).
 
+### Client lifetime
+
+Each of the Event Hubs client types is safe to cache and use as a singleton for the lifetime of the application, which is best practice when events are being published or read regularly. The clients are responsible for efficient management of network, CPU, and memory use, working to keep usage low during periods of inactivity.  Calling either `CloseAsync` or `DisposeAsync` on a client is required to ensure that network resources and other unmanaged objects are properly cleaned up.
+
+### Thread safety
+
+We guarantee that all client instance methods are thread-safe and independent of each other ([guideline](https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-service-methods-thread-safety)). This ensures that the recommendation of reusing client instances is always safe, even across threads.
+
+### Additional concepts
+
+<!-- CLIENT COMMON BAR -->
+[Client options](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/eventhub/Azure.Messaging.EventHubs/samples/Sample02_EventHubsClients.md#configuration) | [Handling failures](#exception-handling) | [Diagnostics](#logging-and-diagnostics) |
+[Mocking](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#mocking)
+<!-- CLIENT COMMON BAR -->
+
 ## Examples
 
 ### Inspect an Event Hub

sdk/eventhub/Azure.Messaging.EventHubs/samples/Sample02_EventHubsClients.md

@@ -24,7 +24,7 @@ The mainstream set of clients provides an approachable onboarding experience for
 
 ## Lifetime
 
-Each of the Event Hubs client types is safe to cache and use for the lifetime of the application, which is best practice when the application publishes or reads events regularly or semi-regularly. The clients are responsible for efficient resource management, working to keep resource usage low during periods of inactivity and manage health during periods of higher use. Calling either the `CloseAsync` or `DisposeAsync` method on a client as the application is shutting down will ensure that network resources and other unmanaged objects are properly cleaned up.
+Each of the Event Hubs client types is safe to cache and use as a singleton for the lifetime of the application, which is best practice when events are being published or read regularly. The clients are responsible for efficient management of network, CPU, and memory use, working to keep usage low during periods of inactivity.  Calling either `CloseAsync` or `DisposeAsync` on a client is required to ensure that network resources and other unmanaged objects are properly cleaned up.
 
 ## Configuration
 

sdk/servicebus/Azure.Messaging.ServiceBus/README.md

@@ -88,6 +88,23 @@ To interact with these resources, one should be familiar with the following SDK
 
 For more concepts and deeper discussion, see: [Service Bus Advanced Features](https://docs.microsoft.com/azure/service-bus-messaging/service-bus-messaging-overview#advanced-features).
 
+### Client lifetime
+
+The `ServiceBusClient`, senders, receivers, and processors are safe to cache and use as a singleton for the lifetime of the application, which is best practice when messages are being sent or received regularly. They are responsible for efficient management of network, CPU, and memory use, working to keep usage low during periods of inactivity. 
+
+These types are disposable and calling either `DisposeAsync` or `CloseAsync` is required to ensure that network resources and other unmanaged objects are properly cleaned up.  It is important to note that when a `ServiceBusClient` instance is disposed, it will automatically close and cleanup any senders, receivers, and processors that were created using it.
+
+### Thread safety
+
+We guarantee that all client instance methods are thread-safe and independent of each other ([guideline](https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-service-methods-thread-safety)). This ensures that the recommendation of reusing client instances is always safe, even across threads.
+
+### Additional concepts
+
+<!-- CLIENT COMMON BAR -->
+[Client options](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#configuring-service-clients-using-clientoptions) | [Handling failures](#exception-handling) | [Diagnostics](#logging-and-diagnostics) |
+[Mocking](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#mocking) 
+<!-- CLIENT COMMON BAR -->
+
 ## Examples
 
 * [Send and receive a message](#send-and-receive-a-message)