enumerator.c: add comment

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42706 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
enumerator.c: rb_check_funcall

* enumerator.c (enumerator_size): use rb_check_funcall() instead of
  respond_to? and call.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42707 b2dd03c8-39d4-4d8f-98ff-823fe69b080e