bug#18024: 24.4.50; doc of `y-or-n-p' misses the main functionality: what it returns

[Drew Adams <drew.adams@oracle.com>]

1. `y-or-n-p' is a *predicate*.  This is missing from the doc string,
because it says nothing about what the function returns if the user
answers `n' instead of `y'.  The doc string *must* tell users that `n'
means return nil.

2. The Elisp manual does not have that failing.  However, to be clear,
both it and the doc string should specify that *whatever* way the user
answers in the affirmative means return `t', and *whatever* s?he answers
in the negative means return `nil'.  Currently the manual specifies the
return value only for `y' and `n'.

In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-06-28 on ODIEONE
Bzr revision: 117431 rgm@gnu.org-20140628015517-eku6hj8mpgcvfnso
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''