patch 8.1.0105: all tab stops are the same

Problem:    All tab stops are the same.
Solution:   Add the variable tabstop feature. (Christian Brabandt,
            closes #2711)
diff --git a/runtime/doc/various.txt b/runtime/doc/various.txt
index 7c8f311..bd2b35e 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -453,6 +453,7 @@
 N  *+title*		Setting the window 'title' and 'icon'
 N  *+toolbar*		|gui-toolbar|
 N  *+user_commands*	User-defined commands. |user-commands|
+B  *+vartabs*		Variable-width tabstops. |'vartabstop'|
 N  *+viminfo*		|'viminfo'|
    *+vertsplit*		Vertically split windows |:vsplit|; Always enabled
 			since 8.0.1118.