Merge topic 'doc-unity-build'

13c8dbd5a6 Help: Document CMAKE_UNITY_BUILD/CMAKE_EXPORT_COMPILE_COMMANDS limitation
ed06d27c7f Help: Extend documentation of CMAKE_UNITY_BUILD variable
1d9155eb93 Help: Improve UNITY_BUILD documentation formatting
8aac65a361 Help: Document CMAKE_UNITY_BUILD in 3.16 release notes

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !3905