Added best practice notes to the CosmosClient docs (Azure#30765)

Author: rinatmini

sdk/cosmos/azure-cosmos/azure/cosmos/aio/_cosmos_client.py

@@ -95,6 +95,12 @@ class CosmosClient(object):  # pylint: disable=client-accepts-api-version-keywor
 
     Use this client to configure and execute requests to the Azure Cosmos DB service.
 
+    Its recommended to maintain a single instance of CosmosClient per lifetime of the application which enables
+        efficient connection management and performance.
+
+    CosmosClient initialization is a heavy operation - don't use initialization CosmosClient instances as
+        credentials or network connectivity validations.
+
     :param str url: The URL of the Cosmos DB account.
     :param credential: Can be the account key, or a dictionary of resource tokens.
     :type credential: Union[str, Dict[str, str], ~azure.core.credentials_async.AsyncTokenCredential]

sdk/cosmos/azure-cosmos/azure/cosmos/cosmos_client.py

@@ -135,6 +135,12 @@ class CosmosClient(object):  # pylint: disable=client-accepts-api-version-keywor
 
     Use this client to configure and execute requests to the Azure Cosmos DB service.
 
+    Its recommended to maintain a single instance of CosmosClient per lifetime of the application which enables
+        efficient connection management and performance.
+
+    CosmosClient initialization is a heavy operation - don't use initialization CosmosClient instances as
+        credentials or network connectivity validations.
+
     :param str url: The URL of the Cosmos DB account.
     :param credential: Can be the account key, or a dictionary of resource tokens.
     :type credential: Union[str, Dict[str, str], ~azure.core.credentials.TokenCredential]