Clarify use for shared_credential_file (elastic#20929)

When starting beats as a service, the PID will be owned by the user that manages the service. This would be root in most cases. Users tend to run tests as non-root, running beats directly (./metricbeat) on the command line. Without the shared_credential_file path the beat checks for credentials under the user's home directory. When starting beats as a service, the home directory of the user managing the service (typically root) is different than what was being used in testing and development, which can be hard to figure out.

x-pack/libbeat/docs/aws-credentials-config.asciidoc

@@ -38,7 +38,8 @@ given, the default profile will be used.
 `shared_credential_file` is optional to specify the directory of your shared
 credentials file. If it's empty, the default directory will be used.
 In Windows, shared credentials file is at `C:\Users\<yourUserName>\.aws\credentials`.
-For Linux, macOS or Unix, the file is located at `~/.aws/credentials`. Please see
+For Linux, macOS or Unix, the file is located at `~/.aws/credentials`. When running as a service, 
+the home path depends on the user that manages the service, so the `shared_credential_file` parameter can be used to avoid ambiguity. Please see
 https://docs.aws.amazon.com/ses/latest/DeveloperGuide/create-shared-credentials-file.html[Create Shared Credentials File]
 for more details.