Make scroll-bar-adjust-thumb-portion customizable, adjust documentation.

* lisp/cus-start.el (all): Add scroll-bar-adjust-thumb-portion.

* src/xterm.c (syms_of_xterm): Adjust documentation for
scroll-bar-adjust-thumb-portion.

lisp/ChangeLog

+2013-01-11  Jan Djärv  <jan.h.d@swipnet.se>
+
+	* cus-start.el (all): Add scroll-bar-adjust-thumb-portion.
+
 2013-01-13  Stefan Monnier  <monnier@iro.umontreal.ca>
 	* jit-lock.el (jit-lock-debug-mode): New minor mode.

lisp/cus-start.el

@@ -513,6 +513,7 @@ since it could result in memory overflow and make Emacs crash."
 	     (x-use-underline-position-properties display boolean "22.1")
 	     (x-underline-at-descent-line display boolean "22.1")
 	     (x-stretch-cursor display boolean "21.1")
+	     (scroll-bar-adjust-thumb-portion windows boolean "24.4")
 	     ;; xselect.c
 	     (x-select-enable-clipboard-manager killing boolean "24.1")
 	     ;; xsettings.c

src/ChangeLog

+2013-01-11  Jan Djärv  <jan.h.d@swipnet.se>
+
+	* xterm.c (syms_of_xterm): Adjust documentation for
+	scroll-bar-adjust-thumb-portion.
+
 2012-12-31  Adam Sjøgren  <asjo@koldfront.dk>
 	* xterm.c (scroll-bar-adjust-thumb-portion): New variable to

src/xterm.c

@@ -10789,10 +10789,12 @@ With MS Windows or Nextstep, the value is t.  */);
 
   DEFVAR_BOOL ("scroll-bar-adjust-thumb-portion",
                scroll_bar_adjust_thumb_portion_p,
-               doc: /* Non-nil means adjust the thumb in the
-scroll bar to be less likely to resize which looks better.
-Set to nil if you want the thumb to fill the whole scroll bar
-when the entire buffer is visible.  */);
+               doc: /* Adjust thumb for overscrolling for Gtk+ and MOTIF.
+Non-nil means adjust the thumb in the scroll bar so it can be dragged downwards
+even if the end of the buffer is shown (i.e. overscrolling).
+Set to nil if you want the thumb to be at the bottom when the end of the buffer
+is shown.  Also, the thumb fills the whole scroll bar when the entire buffer
+is visible.  In this case you can not overscroll.  */);
   scroll_bar_adjust_thumb_portion_p = 1;
 
   staticpro (&last_mouse_motion_frame);