Merge topic 'improve-doc-signatures'

533ebb072b Help: Use signature directive for string command
cd33f461a6 Help: Use signature directive for cmake_language command
74e3c1d313 Utilities/Sphinx: Add a directive to document command signatures
c09b760484 Utilities/Sphinx: Drop commented-out code from cmake domain impl
6e85ad7f8b Help/dev: Add per-directive sections in CMake Documentation Guide

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !8243