Handle help-form in y-or-n-p and use this in find-file-noselect (bug#5423)

Handle help-form in y-or-n-p and use this in find-file-noselect (bug#5423)

* doc/lispref/help.texi (Help Functions): Mention help-form for
read-char-from-minibuffer and y-or-n-p.

* doc/lispref/minibuf.texi (Yes-or-No Queries): Mention help-form
for y-or-n-p.
(Multiple Queries): Mention help-form for read-char-from-minibuffer.

* lisp/files.el (find-file-noselect): Let-bind multi-line help text
to help-form for y-or-n-p.

* lisp/subr.el (read-char-choice): Mention help-form in docstring.
(read-char-from-minibuffer): Mention help-form in docstring.
(y-or-n-p-map): Remove handling of 'help'.
(y-or-n-p): Mention help-form in docstring.
When help-form is non-nil: add help-char to 'prompt', and bind
help-char to help-form-show in composed-keymap.