kbuild: add more comments for KBUILD_NOCMDDEP=1

The cmd-check for KBUILD_NOCMDDEP=1 may not be clear until you see
commit c4d5ee13 ("kbuild: make KBUILD_NOCMDDEP=1 handle empty
built-in.o").

When a phony target (i.e. FORCE) is the only prerequisite, Kbuild
uses a tricky way to detect that the target does not exist.

Add more comments.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>