Merge topic 'doc-add_custom_command'

303c48fbd4 Help: Add add_custom_command example with output consumed by multiple targets
5f2bf6ecc6 Help: Drop misleading add_custom_command "makefile terms" explanation

Acked-by: Kitware Robot <kwrobot@kitware.com>
Rejected-by: BRevzin <barry.revzin@gmail.com>
Merge-request: !8002