Document yank behavior in the right place

Document yank behavior in the right place

* lisp/simple.el (yank): Document the handling of the
`yank-handled-properties' and `yank-excluded-properties' variables,
and the `yank-handler' text property.
(yank-pop): Refer to `yank' now (bug#286)

* lisp/subr.el (insert-for-yank): Refer to `yank' now.
(insert-for-yank-1): Refer to `insert-for-yank' now.

See this thread for discussion:

  https://lists.gnu.org/archive/html/emacs-devel/2016-09/threads.html#00329
  From: Karl Fogel
  To: Emacs Devel
  Subject: Question about intended behavior of 'insert-for-yank-1'.
  Date: Mon, 12 Sep 2016 00:17:14 -0500
  Message-ID: <874m5lr92d.fsf@red-bean.com>