common.mk: clean more files

* common.mk (distclean-local): clean fake file.

* common.mk (realclean-local): clean generated files under srcdir.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48681 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
array.c (rb_ary_plus): in documentation, added note about
	  inefficiency of repeated += operations. [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48682 b2dd03c8-39d4-4d8f-98ff-823fe69b080e