Help: Mention add_dependencies in add_custom_target DEPENDS option

This will direct readers looking to add target-level dependencies.
Help: Document MAIN_DEPENDENCY limitation in add_custom_command

Specify explicitly that at most one custom command may use a given
source file as its main dependency.
Merge topic 'doc-custom-command-depends' into next

953d34fd Help: Document MAIN_DEPENDENCY limitation in add_custom_command
9859042c Help: Mention add_dependencies in add_custom_target DEPENDS option