Update docs for multiprocessing

Our docs already give advice about multithreading, but did not give
any advice about multiprocessing. This explicitly calls out that the
reccomendations for multithreading and multiprocessing are the same.

docs/source/guide/resources.rst

@@ -198,9 +198,9 @@ keyword arguments. Examples of waiters include::
     instance.wait_until_running()
 
 
-Multithreading
---------------
-It is recommended to create a resource instance for each thread in a multithreaded application rather than sharing a single instance among the threads. For example::
+Multithreading / Multiprocessing
+--------------------------------
+It is recommended to create a resource instance for each thread / process in a multithreaded or multiprocess application rather than sharing a single instance among the threads / processes. For example::
 
     import boto3
     import boto3.session