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.
• eliminate redundant implication information from short descriptions.

Fixes #23895 (closed).