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.

runtime/doc/options.txt

@@ -7758,6 +7758,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	the text to be displayed.  Use "%1T" for the first label, "%2T" for
 	the second one, etc.  Use "%X" items for closing labels.
 
+	When changing something that is used in 'tabline' that does not
+	trigger it to be updated, use |:redrawtabline|.
+
 	Keep in mind that only one of the tab pages is the current one, others
 	are invisible and you can't jump to their windows.
 

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 @@ CTRL-L			Clear and redraw the screen.  The redraw may happen
 			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