users-guide: Clarify language extension documentation

Over the years the users guide's language extension documentation has
gone through quite a few refactorings. In the process some of the
descriptions have been rendered non-sensical. For instance, the
description of `NoImplicitPrelude` actually describes the semantics of
`ImplicitPrelude`.

To fix this we:

 * ensure that all extensions are named in their "positive" sense (e.g.
   `ImplicitPrelude` rather than `NoImplicitPrelude`).
 * rework the documentation to avoid flag-oriented wording
   like "enable" and "disable"
 * ensure that the polarity of the documentation is consistent with
   reality.

Fixes #23895.