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

[Alex]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Mon, 12 Sep 2016 10:19:42 -0700 (PDT)
>> From: Drew Adams <drew.adams@oracle.com>
>> Cc: drew.adams@oracle.com, 20253@debbugs.gnu.org
>>
>> Questions about this arise from end users.
>
> I didn't see any.  Please invite those users to ask the questions here
> or on emacs-devel.  Maybe I will change my mind after hearing them.
>

My curiosity about the meaning of a cons cell priority is what pushed me
to find and respond to this bug, so I guess that would count as one
question.

Since the region overlay is a very common overlay, I think it is
important enough to at least have a passing statement about its priority
value type in the documentation. Even something like "For example, some
overlays use a cons cell priority (PRIMARY . SECONDARY), where SECONDARY
is used as a tie-breaker if the PRIMARY priorities and boundaries of the
overlays are equal." would be nice.