Clarify the doc strings of mouse-position and set-mouse-position.

 src/frame.c (Fmouse_position, Fset_mouse_position): Clarify the
 units in which the position is measured.  (Bug#18493)

src/ChangeLog

 2014-09-18  Eli Zaretskii  <eliz@gnu.org>
 
+	* frame.c (Fmouse_position, Fset_mouse_position): Clarify the
+	units in which the position is measured.  (Bug#18493)
+
 	* xdisp.c (redisplay_internal): Force redisplay of all windows
 	that show a buffer whose narrowing has changed.  (Bug#18490)
 

src/frame.c

@@ -1524,9 +1524,9 @@ The functions are run with one argument, the frame to be deleted.  */)
 
 DEFUN ("mouse-position", Fmouse_position, Smouse_position, 0, 0, 0,
        doc: /* Return a list (FRAME X . Y) giving the current mouse frame and position.
-The position is given in character cells, where (0, 0) is the
-upper-left corner of the frame, X is the horizontal offset, and Y is
-the vertical offset.
+The position is given in canonical character cells, where (0, 0) is the
+upper-left corner of the frame, X is the horizontal offset, and Y is the
+vertical offset, measured in units of the frame's default character size.
 If Emacs is running on a mouseless terminal or hasn't been programmed
 to read the mouse position, it returns the selected frame for FRAME
 and nil for X and Y.
@@ -1609,9 +1609,10 @@ Coordinates are relative to the frame, not a window,
 so the coordinates of the top left character in the frame
 may be nonzero due to left-hand scroll bars or the menu bar.
 
-The position is given in character cells, where (0, 0) is the
-upper-left corner of the frame, X is the horizontal offset, and Y is
-the vertical offset.
+The position is given in canonical character cells, where (0, 0) is
+the upper-left corner of the frame, X is the horizontal offset, and
+Y is the vertical offset, measured in units of the frame's default
+character size.
 
 This function is a no-op for an X frame that is not visible.
 If you have just created a frame, you must wait for it to become visible