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.