doc/Makefile: add docclean rule

This allows to clean the doc directory from the parent project dir.
The command:
make -C doc clean

does not work as long as the paths specified in doc/Makefile are relative
to the parent dir.

Also it is consitent with "testclean" and "distclean" targets.

doc/Makefile

@@ -93,6 +93,8 @@ uninstall: uninstall-man
 uninstall-man:
 	$(RM) $(addprefix "$(MANDIR)/man1/",$(ALLMANPAGES))
 
+docclean: clean
+
 clean::
 	$(RM) $(TXTPAGES) doc/*.html doc/*.pod doc/*.1 doc/*.3 $(CLEANSUFFIXES:%=doc/%) doc/avoptions_*.texi
 	$(RM) -r doc/doxy/html