patch 8.2.3969: value of MAXCOL not available in Vim script

Problem:    Value of MAXCOL not available in Vim script.
Solution:   Add v:maxcol. (Naohiro Ono, closes #9451)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 3839207..dca886e 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2161,6 +2161,9 @@
 		expressions is being evaluated.  Read-only when in the
 		|sandbox|.
 
+						*v:maxcol* *maxcol-variable*
+v:maxcol	Maximum line length.
+
 					*v:mouse_win* *mouse_win-variable*
 v:mouse_win	Window number for a mouse click obtained with |getchar()|.
 		First window has number 1, like with |winnr()|.  The value is