"make clobber" should remove modindex.html as well.

Added realclean and distclean as synonyms for clobber.
diff --git a/Doc/html/Makefile b/Doc/html/Makefile
index 538d68f..ce0baa0 100644
--- a/Doc/html/Makefile
+++ b/Doc/html/Makefile
@@ -128,8 +128,8 @@
 clean:
 	rm -rf @webchecker.pickle
 
-clobber: clean
-	rm -rf index.html api/ doc/ ext/ lib/ mac/ ref/ tut/
+distclean realclean clobber: clean
+	rm -rf index.html modindex.html api/ doc/ ext/ lib/ mac/ ref/ tut/
 
 
 # This is really ugly, but we're not dependent on $(RELEASE), which isn't