docs: add note about multiprocessing usage (#26)

Author: crwilcox

docs/index.rst

@@ -1,5 +1,12 @@
 .. include:: README.rst
 
+.. note::
+
+   Because this 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`.
 
 Using the API
 -------------

noxfile.py

@@ -141,7 +141,7 @@ def docs(session):
     """Build the docs for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark")
+    session.install("sphinx<3.0.0", "alabaster", "recommonmark")
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(