Document `scroll-bar-adjust-thumb-portion'.

doc/emacs/ChangeLog

+2013-12-22  Xue Fuqiao  <xfq.free@gmail.com>
+
+	* frames.texi (Scroll Bars): Document `scroll-bar-adjust-thumb-portion'.
+
 2013-12-21  Chong Yidong  <cyd@gnu.org>
 
 	* indent.texi (Indentation Commands): Document C-x TAB changes.

doc/emacs/frames.texi

@@ -930,6 +930,17 @@ or disable the scroll bars (@pxref{Resources}).  To control the scroll
 bar width, change the @code{scroll-bar-width} frame parameter
 (@pxref{Frame Parameters,,, elisp, The Emacs Lisp Reference Manual}).
 
+@vindex scroll-bar-adjust-thumb-portion
+@cindex overscrolling
+If you're using Emacs on X (with GTK+ or Motif), you can customize the
+variable @code{scroll-bar-adjust-thumb-portion} to control
+@dfn{overscrolling} of the scroll bar, i.e. dragging the thumb down even
+when the end of the buffer is visible.  If its value is
+non-@code{nil}, the scroll bar can be dragged downwards even if the
+end of the buffer is shown; if @code{nil}, the thumb will be at the
+bottom when the end of the buffer is shown.  You can not over-scroll
+when the entire buffer is visible.
+
 @node Drag and Drop
 @section Drag and Drop
 @cindex drag and drop

etc/NEWS

@@ -179,6 +179,7 @@ As a command, it was a special case of `C-x 2' (`split-window-below'),
 and as such superfluous.  After being reimplemented in Lisp, its
 interactive form was mistakenly retained.
 
++++
 *** New option `scroll-bar-adjust-thumb-portion'.
 Available only on X, this option allows to control over-scrolling
 using the scroll bar (i.e. dragging the thumb down even when the end