runtime(doc): Makefile does not clean up all temporary files

Also remove (automatically generated) tags-* (made by make vimtags) and
vim-stylesheet.css (which is produced by vim2html.pl).

related: #16061

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

runtime/doc/Makefile

@@ -93,7 +93,7 @@ test_urls:
 	$(VIMPROG) --clean -S test_urls.vim
 
 clean:
-	-rm -f doctags *.html tags.ref
+	-rm -f doctags *.html tags.ref tags-* vim-stylesheet.css
 
 # These files are in the extra archive, skip if not present