kbuild: Update documentation of clean-files and clean-dirs
kbuild: Update documentation of clean-files and clean-dirs

Commit a16c5f99 (kbuild: Fix removal of the debian/ directory) slightly
changed the processing of the clean-files and clean-dirs variables.
Also, use a current real-world example of clean-files usage.

Signed-off-by: Michal Marek <0c835876a532a476f30fdb1f6e963bae6c5a1a04@suse.cz>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs