(Horizontal Scrolling): Add an index entry for automatic-hscrolling.

lispref/windows.texi

@@ -1542,6 +1542,7 @@ reduce the net horizontal scroll to zero.  There is no limit to how far
 left you can scroll, but eventually all the text will disappear off the
 left edge.
 
+@vindex automatic-hscrolling
   In Emacs 21, redisplay automatically alters the horizontal scrolling
 of a window as necessary to ensure that point is always visible, if
 @code{automatic-hscrolling} is set.  However, you can still set the