(Optional Mode Line): Document line-number-display-limit-width.

man/display.texi

@@ -666,13 +666,23 @@ before the buffer percentage @var{pos}, with the letter @samp{L} to
 indicate what it is.  @xref{Minor Modes}, for more information about
 minor modes and about how to use this command.
 
+@cindex narrowing, and line number display
+  If you have narrowed the buffer (@pxref{Narrowing}), the displayed
+line number is relative to the accessible portion of the buffer.
+
 @vindex line-number-display-limit
   If the buffer is very large (larger than the value of
 @code{line-number-display-limit}), then the line number doesn't appear.
 Emacs doesn't compute the line number when the buffer is large, because
-that would be too slow.  Set it to @code{nil} to remove the limit.  If
-you have narrowed the buffer (@pxref{Narrowing}), the displayed line
-number is relative to the accessible portion of the buffer.
+that would be too slow.  Set it to @code{nil} to remove the limit.
+
+@vindex line-number-display-limit-width
+  Line-number computation can also be slow if the lines in the buffer
+are too long.  For this reason, Emacs normally doesn't display line
+numbers if the average width, in characters, of lines near point is
+larger than the value of the variable
+@code{line-number-display-limit-width}.  The default value is 200
+characters.
 
 @cindex Column Number mode
 @cindex mode, Column Number