bug#51927: 27.2; Rename `highlight-nonselected-windows'

[Drew Adams]

This variable seems poorly named, in more ways than one.  Could we
consider renaming it (by aliasing and perhaps deprecating the old name)?

Maybe use a name such as `show-active-region-everywhere'.  I won't fight
over the name, but I think it should have "region" in it, and ideally
"active" as well.

Not only is the name poor, but the doc is also less than ideal.

1. This var has no effect if `transient-mark-mode' is disabled.  The doc
should say something about it being relevant only for t-m-mode.  It
should say that when that's disabled the region is never "active", and
so it is never highlighted anywhere.

2. This is not really about highlighting windows.  (Yes, the
highlighting occurs in windows, of course.  But it's the active region
that's highlighted.)

3. The name says nothing about what's most important: "region".  This is
about showing (highlighting) the _active region_ in _all_ windows.

4. The doc should explain the alternative behavior, when the var is nil:
the active region is highlighted only in the selected window.

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19042
System Description: Microsoft Windows 10 Pro (v10.0.2009.19042.1288)