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

[Eli Zaretskii <eliz@gnu.org>]

> From: Alex <agrambot@gmail.com>
> Cc: Drew Adams <drew.adams@oracle.com>,  20253@debbugs.gnu.org
> Date: Wed, 21 Sep 2016 12:11:08 -0600
> 
> 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.

I hoped to hear some practical reasons for this, not just curiosity.
Like practical use cases where knowing that internal detail (as
opposed to using the documented methods of comparing priorities etc.)
is imperative for that use case.

People often ask questions here out of curiosity about the display
engine's inner workings, for example, and I try to answer them as best
as I can.  But that doesn't mean all I write here in those discussions
should be in the manual.

IOW, the need to have some internal detail described in the manual
(which implies we will have to update and maintain it for the years to
come) should have more important reasons than just curiosity.