kbuild: update help in top level Makefile

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Makefile b/Makefile
index fc67adc..0d4a4dc 100644
--- a/Makefile
+++ b/Makefile
@@ -1060,6 +1060,7 @@
 	@echo  'Cleaning targets:'
 	@echo  '  clean		  - remove most generated files but keep the config'
 	@echo  '  mrproper	  - remove all generated files + config + various backup files'
+	@echo  '  distclean	  - mrproper + patch files'
 	@echo  ''
 	@echo  'Configuration targets:'
 	@$(MAKE) -f $(srctree)/scripts/kconfig/Makefile help