User's guide: Improve docs for -Wall

previously it would list the warnings not enabled by -Wall. That’ unnecessary round-about and was out of date. So let's just name the relevant warnings (based on compiler/GHC/Driver/Flags.hs).

One could also try to generate these lists, to keep them fully accurate. The tool at https://github.com/haskellfoundation/error-message-index/tree/main/message-index/warning-sets does more or less that using the GHC API, and can probably adapted.

Fixes #22076