docs: add clarification to batch module

[cojenco]

This PR is intended to:

• Clarify limited Batch support in the python client
• Provide guidance on compatible use cases with batch requests
• Add comment in batch sample

The current batch design does not support library methods whose return values depend on the response payload. Adding such support would be a huge undertaking, and is not prioritized at the moment. Incompatible use cases include but are not limited to:

• error handling (i.e. blob.exists() or on_error callable in bucket.delete_blobs())
• return value is extracted from the response payload (blob.rewrite() returns a 3-tuple, (rewrite_token, bytes_rewritten, object_size))