Docs: Clarify that input is one time only on communicate()

Author: gpshead

Doc/library/subprocess.rst

@@ -831,7 +831,9 @@ Instances of the :class:`Popen` class have the following methods:
 
    If the process does not terminate after *timeout* seconds, a
    :exc:`TimeoutExpired` exception will be raised.  Catching this exception and
-   retrying communication will not lose any output.
+   retrying communication will not lose any output.  Supplying *input* to a
+   subsequent post-timeout :meth:`communicate` call is in undefined behavior
+   and may become an error in the future.
 
    The child process is not killed if the timeout expires, so in order to
    cleanup properly a well-behaved application should kill the child process and