Document thread-safety of library

[BrandonY]

After reading through the docs, it is unclear to me whether I should expect this code to work:

from google.cloud import storage
import thread

client = storage.Client()
for blob in bucket.list_blobs():
  thread.start_new_thread( blob.download_to_filename, '/tmp/' + blob.name)

[tseaver]

google-cloud-storage has been based on requests since #3674, released with google-cloud-storage 1.3.0 just about a year ago.

I'm leaving this open as a docs bug so that we add a note to that effect.

[BrandonY]

Sorry, you last me. Does being based on requests imply that google-cloud-storage is thread-safe, and the code example above should work fine?

[tseaver]

Yes to both questions.

[tseaver]

@BrandonY Here is the text I am planning to add to the client page:

.. note::

   Becuase the :class:`~google.cloud.storage.client.Client` uses the
   third-party :mod:`requests` library by default, it should be safe to
   share instances across threads.  In multiprocessing scenarious, best
   practice is to create client instances *after* 
   :class:`multiprocessing.Pool` or :class:`multiprocessing.Process` invokes
   :func:`os.fork`.

Does that address your need?

[tseaver]

@BrandonY Please follow up via #5763, if needed.