Mention the "context manager" keyword in concurrent.futures documentation

I was searching for context manager-related part part of this documentation
and I couldn't find anything because the words "context manager"
weren't actually used anywhere.

I think this will help with discoverability of this feature.

Author: jstasiak

Doc/library/concurrent.futures.rst

@@ -94,10 +94,10 @@ Executor Objects
       executor has started running will be completed prior to this method
       returning. The remaining futures are cancelled.
 
-      You can avoid having to call this method explicitly if you use the
-      :keyword:`with` statement, which will shutdown the :class:`Executor`
-      (waiting as if :meth:`Executor.shutdown` were called with *wait* set to
-      ``True``)::
+      You can avoid having to call this method explicitly if you use the executor
+      as a :term:`context manager` via the  :keyword:`with` statement,
+      which will shutdown the :class:`Executor` (waiting as if
+      :meth:`Executor.shutdown` were called with *wait* set to ``True``)::
 
          import shutil
          with ThreadPoolExecutor(max_workers=4) as e: