patch 8.1.0706: tabline is not always redrawn

Problem:    Tabline is not always redrawn when something that is used in
            'tabline' changes.
Solution:   Add ":redrawtabline" so that a plugin can at least cause the
            redraw when needed.
diff --git a/runtime/doc/various.txt b/runtime/doc/various.txt
index 67acc64..37b7074 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -1,4 +1,4 @@
-*various.txt*   For Vim version 8.1.  Last change: 2018 Dec 13
+*various.txt*   For Vim version 8.1.  Last change: 2019 Jan 08
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -30,6 +30,11 @@
 			includes an item that doesn't cause automatic
 			updating.
 
+						*:redrawt* *:redrawtabline*
+:redrawt[abline]	Redraw the tabline.  Useful to update the tabline when
+			'tabline' includes an item that doesn't trigger
+			automatic updating.
+
 							*N<Del>*
 <Del>			When entering a number: Remove the last digit.
 			Note: if you like to use <BS> for this, add this