(Tool Bar): Add tool-bar-border.

lispref/keymaps.texi

@@ -2496,6 +2496,15 @@ The value is an integer, a number of pixels.  The default is 4.
 @defvar tool-bar-button-relief
 This variable specifies the shadow width for tool bar items.
 The value is an integer, a number of pixels.  The default is 1.
+@end defvar
+
+@tindex tool-bar-border
+@defvar tool-bar-border
+This variable specifies the height of the border drawn below the tool
+bar area.  An integer value specifies height as a number of pixels.
+If the value is one of @code{internal-border-width} (the default) or
+@code{border-width}, the tool bar border height corresponds to the
+corresponding frame parameter.
 @end defvar
 
   You can define a special meaning for clicking on a tool bar item with