Features: Add missing variable reference in docs.
Help: Add a manual for compiler feature control.

Link to it from the documentation of related properties, variables
and commands.

Extend the cmake-developer(7) documentation with notes on
extending feature support for compilers.
Merge topic 'compile-features-manual' into next

1b6f9fb6 Help: Add a manual for compiler feature control.
b4d33f64 Features: Add missing variable reference in docs.