Rollup merge of #119282 - Urgau:check-cfg-rework-unstable-doc, r=JohnTitor

Rework and improve the unstable documentation of check-cfg

This PR rework and improve the unstable documentation of the check-cfg feature.

The goal is to have a simpler to understand documentation with examples that are more practical and less theoretical. As well as making the documentation more explicit about the whereabouts of the feature.

`@rustbot` label +T-compiler +F-check-cfg

Author: GuillaumeGomez