Move enum.one? documentation before the relevant method.

* enum.c (enum_one): Move enum.one? documentation before the
  relevant method.  Submitted by @vipulnsward. [Fixes GH-687]
  https://github.com/ruby/ruby/pull/687

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47086 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Oops, maybe it's too late, but add [DOC] to the last entry.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47087 b2dd03c8-39d4-4d8f-98ff-823fe69b080e