Merge topic 'scope-doc-tweaks'

bbf3ed5422 Help: Explain block() policy-scope rules more clearly
2f319c6484 Help: Remove "this command" ambiguity in set() docs
9eecdc7ab7 Help: Use signature directive for cmake_policy()

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !8761