bug#56393: Actually fix the long lines display bug

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 20 Jul 2022 17:33:00 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, larsi@gnus.org, 56393@debbugs.gnu.org
> 
> I also made a few (final?) tweaks.  Please tell me if the feature is ready 
> to land on master, or if further improvements are needed.

Thanks.

I think the code is ready to be merged, but what about the
documentation?  The branch currently removes all traces of the
facilities we previously documented as possible remedies for poor
performance in these cases.  However, we do know that in some
situations redisplay and related commands are still very slow.  The
NEWS entry on the branch does say that font-lock is one reason for the
slowdown.  So at least that part should be in the manual, perhaps in
the form of a shorter description of so-long mode (because one thing
it does is disable font-lock).  We can remove that later, if and when
these problems and any others we find are also resolved.

IOW, I think we still don't have enough data and experience to decide
that all those other measures will never be needed, so maybe it is too
early to delete their docs.