[1.5.x] Warned that `request_finished` isn't sent by some buggy setups.

Older versions of uWSGI and Sentry's middleware do not adhere to
the WSGI spec and cause the `request_finished` signal to never
fire. Added notes to the appropriate places in the docs.

Fixed #20537.

Backport of 3ce1d30.

Conflicts:
	docs/howto/deployment/wsgi/index.txt