bug#20253: 24.4; Document `region' face behavior changes, overlay, priority

[Drew Adams]

Since Emacs 24.3, if I recall correctly, face `region' uses an overlay.
This produces changes in the user-observable behavior - that's why the
change was made.

But the NEWS for 24.3 and for 24.4 do not mention anything about this,
AFAICT.  Not only does NEWS not seem to say anything about `region'
using an overlay, but it does not even mention the observable behavior
differences that this produces.

And the Elisp manual is likewise silent on this change.

Please tell users that such a change was made.  That's what NEWS is for.

And please tell users, in the Elisp manual, what the default priority of
the overlay is.  Testing shows a priority value of (nil . 100), which is
not even a documented form that an overlay priority can take.  But even
if that is what the value needs to be, for internal reasons, at least
tell users what it is.

And if, for some reason, users are supposed not to change the priority,
then say so, in the doc.  And if there is no such restriction or
recommendation, then let them know, if they should be assigning a
priority value that is a cons, such as (nil . NUMBER).

IOW, please communicate with your users about such changes: what they
mean for users (what behavior differences they can expect), and how they
can manipulate/customize the behavior.

In GNU Emacs 24.4.1 (i686-pc-mingw32)
 of 2014-10-24 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr'