Improve documentation of new behavior of 'M-y'

Improve documentation of new behavior of 'M-y'

* lisp/minibuffer.el (minibuffer-local-map): Switch the order of
'\r' and '\n' bindings.
* lisp/simple.el (yank-pop, read-from-kill-ring)
(yank-from-kill-ring): Doc fixes.

* doc/emacs/search.texi (Isearch Yank):
* doc/emacs/killing.texi (Yanking): Improve the description of the
new functionality of 'M-y'.
* doc/lispref/text.texi (Yank Commands): Remove inaccurate
description of 'M-y' in Isearch.

* etc/NEWS: Improve the wording of 'M-y' entry.