bug#29597: 26.0.90; line-number-display-width returns incorrect width

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Fri, 8 Dec 2017 10:37:09 -0500
> Cc: Paul Rankin <hello@paulwrankin.com>, 29597@debbugs.gnu.org
> 
> (line-number-display-width) => 2
> (line-number-display-width t) => the equivalent in pixels

Actually, equivalent to what?  If to 2 (not 4), then this value would
be useless, I think.  And if it's equivalent to 4, then it should be
(line-number-display-columns t), right?