Documentation: kbuild: Improve if_changed documentation

  - These days if_changed is used with many more commands than ld,
    objcopy, and gzip, hence add an ellipsis,
  - Any target that utilises if_changed must be listed in $(targets),
    so it needs an assignment to "targets", not "target".

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>