Clarify use for shared_credential_file (elastic#20929) (elastic#21082)

(cherry picked from commit f2c9616)

Co-authored-by: Kostas Botsas <kostas.botsas@elastic.co>

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.