Chunk tolerance annotation in streaming completion docs

[auto-d]

Why are these changes needed?

Minor documentation changes requested by @ekzhu in #5078.

The user guide recommends users of the create_stream method (in the chat completion client classes) set the include_usage option to ensure final token usage is sent by the server when the completion streaming finishes. However, the final chunk sent from the server containing the usage information is empty, and this fails a check for empty chunks in _openai_client.py. To retrieve usage and avoid inducing this error, users can adjust the chunk tolerance (max_consecutive_empty_chunk_tolerance) when invoking the streaming completion. This PR adjusts the API docs and includes a suggested revision for the user guide as well.

Related issue number

#5078

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
  • not applicable
• I've made sure all auto checks have passed
  • github checks passing, unclear if there are other hurdles.

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[ekzhu]

Use tip directive: https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#directives

[ekzhu]

Also, apologize for the confusion in my comment in a different issue. I think this should go to the API doc of create_stream method in the BaseOpenAIChatCompletionClient.