bug#15456: 24.3.50; doc string of `yes-or-no-p'

[Drew Adams]

The doc string does not say what value is returned if the user types
`no', which is arguably more important information that saying, as it
does, that the function "returns t if the answer is yes."

Say clearly what the return value is in each case: "yes" (t) and "no"
(nil).

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-09-20 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'