Doc: Recommend including moc-generated files

Previously, we mentioned the including moc_foo.cpp files is possible but
not necessary with CMake. This frames the situation as if there's no
benefit of including moc_foo.cpp files with CMake. That is wrong.

Recommend the inclusion regardless of the build system.

Pick-to: 6.12
Change-Id: Id086f9632268947059b17d6b9a6fba1883a3b4ea
Reviewed-by: Kai Köhne <kai.koehne@qt.io>