Incomplete docstrings for the use_legacy_flow_control parameter

#231 added the `use_legacy_flow_control` parameter to subscriber client and documented it, but it did not document its type and and the default value in all places ([example](https://googleapis.dev/python/pubsub/latest/subscriber/api/client.html#google.cloud.pubsub_v1.subscriber.client.Client.subscribe) - missing from the parameter list). Docstrings should be updated accordingly.