Firestore: Add docs regarding Client threadsafety

[danielx]

Is your feature request related to a problem? Please describe.
There is no mention in the current docs if a google.cloud.firestore.Client() can safely be shared among threads.

Describe the solution you'd like
Some docs explaining if a Client can safely be reused among threads.

[tseaver]

For example, see storage/docs/index.rst. We should do likewise for all clients.