Update concurrent.futures.rst (GH-14061)

Update concurrent.futures.rst (GH-14061)

This PR adds missing details in the [`concurrent.futures`](https://docs.python.org/3/library/concurrent.futures.html) documentation:

* the mention that `Future.cancel` also returns `False` if the call finished running;
* the mention of the states for `Future` that did not complete: pending or running.
(cherry picked from commit 431478d5d74d880692817323198b9605af972fa5)

Co-authored-by: Géry Ogam <gery.ogam@gmail.com>