Issue #17273: Clarify that pool methods can only be used by parent process.

Doc/library/multiprocessing.rst

@@ -284,6 +284,9 @@ For example::
            print(result.get(timeout=1))       # prints "100" unless your computer is *very* slow
            print(pool.map(f, range(10)))      # prints "[0, 1, 4,..., 81]"
 
+Note that the methods of a pool should only ever be used by the
+process which created it.
+
 
 Reference
 ---------
@@ -1665,6 +1668,9 @@ with the :class:`Pool` class.
    *initializer* is not ``None`` then each worker process will call
    ``initializer(*initargs)`` when it starts.
 
+   Note that the methods of the pool object should only be called by
+   the process which created the pool.
+
    .. versionadded:: 3.2
       *maxtasksperchild* is the number of tasks a worker process can complete
       before it will exit and be replaced with a fresh worker process, to enable