bpo-28556: Update the opening note in typing docs (GH-16204)

bpo-28556: Update the opening note in typing docs (GH-16204)

This PR replaces the old note mentioning that `typing` is a provisional module with a new one mentioning types are not enforced at runtime. I am not sure if there was any official announcement about making `typing` non-provisional, but _de-facto_ no new features were added during Python 3.7, and no backwards incompatible changes were made except for few small things that were considered bugs.
(cherry picked from commit 81528ba2e81c39f4d6bca5b785e818c7d08b8501)

Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>