*common.mk: add description about "make clean".

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@15276 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -808,4 +808,5 @@ help:
 	@echo '  test-all:      all ruby tests'
 	@echo '  install:       install all ruby distributions'
 	@echo '  install-nodoc: install without rdoc'
-	@echo '  golf:          make goruby'
+	@echo '  clean:         clean built objects'
+	@echo '  golf:          for golfers'