merge revision(s) 57770:

    enum.c: documentation for Enumerable#sort_by

    * enum.c: [DOC] improve structure of docs for Enumerable#sort_by,
      adopt explanation of the comparison block from Array#sort_by,
      drop mention of 1.8, fix typos.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e