Document new mouse-highlight variable and associated behaviour.

etc/NEWS

@@ -55,6 +55,11 @@ of text properties, overlays, and widgets at point, and lets you get
 more information about them, by clicking on mouse-sensitive areas or
 moving there and pressing RET.
 
+** The highlight shown when the mouse is over a mouse-sensitive areas
+is now cleared when you use the keyboard.  The highlight is shown
+again when you move the mouse.  You can control this behaviour and
+disable mouse highlight entirely using the new variable `mouse-highlight'.
+
 ** font-lock: in modes like C and Lisp where the fontification assumes that
 an open-paren in column 0 is always outside of any string or comment,
 font-lock now highlights any such open-paren-in-column-zero in bold-red