bug#18088: 24.4.50; `region-extract-function': (1) NEWS, (2) return value

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

Variable `region-extract-function' was apparently added in Emacs 24.4.
It is used in several key places that affect killing or deleting text.

1. It should be called out in NEWS, saying what user-visible behavior
   its presence changes.

2. The doc string points out that, depending on the value of its
   argument, its return value is (a) significant and specified or (b)
   insignificant and undefined.

   This is the first time I have seen this for an Emacs function
   distributed with GNU Emacs.  Certainly it is the first time I've
   seen such odd behavior documented.

   This does NOT seem like the kind of thing we should be doing - let
   alone *encouraging* by defining a variable whose value will be
   user-defined functions that have such twisted behavior.

   Is this unusual design necessary?  Seems to be sloppy, error prone,
   and an invitation to bad programming habits by users.

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''