kbuild: document INSTALL_MOD_PATH in 'make help'

Signed-Off-By: Bodo Eggert <7eggert@gmx.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1062,7 +1062,7 @@ help:
 	@echo  '  all		  - Build all targets marked with [*]'
 	@echo  '* vmlinux	  - Build the bare kernel'
 	@echo  '* modules	  - Build all modules'
-	@echo  '  modules_install - Install all modules'
+	@echo  '  modules_install - Install all modules to INSTALL_MOD_PATH (default: /)'
 	@echo  '  dir/            - Build all files in dir and below'
 	@echo  '  dir/file.[ois]  - Build specified target only'
 	@echo  '  dir/file.ko     - Build module including final link'