Added missing docstrings in admin/client.py

[lukalafaye]

Added missing doc strings for the following methods:

_convert_new_topic_request
list_topics
describe_topics
describe_cluster
_convert_describe_acls_response_to_acls
_convert_create_acls_resource_request_v0
_convert_create_acls_resource_request_v1
_convert_create_acls_response_to_acls
_convert_delete_acls_resource_request_v0
_convert_delete_acls_resource_request_v1
_convert_delete_acls_response_to_matching_acls
_convert_describe_config_resource_request
_convert_alter_config_resource_request
_convert_create_partitions_request
_convert_delete_groups_response
_delete_consumer_groups_send_request
_wait_for_futures

[dpkp]

Can we use the same style docstrings as in the rest of the codebase? I'm not sure how admin code got started with this format. See https://google.github.io/styleguide/pyguide.html#383-functions-and-methods (though we've got Arguments instead of Args)