Improve the documentation of M-n for entering file names

Improve the documentation of M-n for entering file names

* lisp/files.el (find-file, find-file-other-window)
(find-file-other-frame): Mention file-name-at-point-functions in
the doc string.  Reported by Florian Weimer <fw@deneb.enyo.de> in
http://lists.gnu.org/archive/html/emacs-devel/2017-11/msg00224.html.

* doc/emacs/mini.texi (Minibuffer History): Document
file-name-at-point-functions and its effect on M-n when typing
file names in the minibuffer.
* doc/emacs/files.texi (File Names):
* doc/emacs/mini.texi (Minibuffer File): Add a cross-reference to
"Minibuffer History", where special features of M-n regarding
files are described.