docs(firestore): clarify client threadsafety (#9254)

* add doc for thread safety

* changes in document as recommended

* change in document description.

Author: HemangChothani

packages/google-cloud-firestore/docs/index.rst

@@ -1,5 +1,12 @@
 .. include:: README.rst
 
+.. note::
+
+   Because the firestore client uses :mod:`grpcio` library, it is safe to
+   share instances across threads. In multiprocessing scenarios, the best
+   practice is to create client instances *after* the invocation of
+   :func:`os.fork` by :class:`multiprocessing.Pool` or
+   :class:`multiprocessing.Process`.
 
 API Reference
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