Re: master d8da850: Document cairo-related fallout to font-backend settings

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Mon, 20 Jan 2020 22:05:23 +0100, Stefan Kangas <stefan@marxist.se> said:

    Stefan> Robert Pluim <rpluim@gmail.com> writes:
    >> +Note also that 'FontBackend' settings in .Xdefaults or .Xresources, or
    >> +'font-backend' frame parameter settings in your init files, may need
    >> +to be adjusted, as 'xft' is no longer a valid backend when using
    >> +Cairo.  Use either 'ftcr' or 'ftcrhb' instead for FreeType and
    >> +FreeType + HarfBuzz respectively.  'x' is still a valid backend.

    Stefan> Should we say that FreeType + Harfbuzz is probably the correct choice
    Stefan> for most users, and perhaps also why?  My concern is that people might
    Stefan> default to the shorter option and we could see unnecessary bug reports
    Stefan> about incorrect composition, etc.

The correct choice depends on how their Emacs has been built; ftcr and
ftcrhb are mutually exclusive [1](see my earlier update to the elisp
manual). I guess we could say something like this:

diff --git a/etc/NEWS b/etc/NEWS
index 6cc392970f..55cf59283c 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -34,9 +34,9 @@ your font settings.
 Note also that 'FontBackend' settings in .Xdefaults or .Xresources, or
 'font-backend' frame parameter settings in your init files, may need
 to be adjusted, as 'xft' is no longer a valid backend when using
-Cairo.  Use either 'ftcr' or 'ftcrhb' instead for FreeType and
-FreeType + HarfBuzz respectively.  'x' is still a valid backend.
-
+Cairo.  Use 'ftcrhb' if your Emacs was built with HarfBuzz support,
+and 'ftcr' otherwise.  You can this determine by checking
+'system-configuration-features'.  'x' is still a valid backend.
 
 ---
 ** The ftx font backend driver has been removed.

Footnotes:
[1]  My master plan to remove as many different variations as possible
     seems to get shot down every time I suggest it :-)