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/options.txt b/runtime/doc/options.txt
index 60f01de..008926f 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7758,6 +7758,9 @@
 	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.