Merge topic 'doc-policy-default'

69b70968bd Help: Suggest CMAKE_POLICY_DEFAULT_CMP0126 in CMP0126 docs
5b1c24255f Help: Suggest CMAKE_POLICY_DEFAULT_CMP0077 in CMP0077 docs
c44636a89b Help: Document more use cases for CMAKE_POLICY_DEFAULT_CMPNNNN

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !6756