display: Update docstring for `WindowProperties::set_mouse_mode()`

[skip ci]

Fixes #1307

panda/src/display/windowProperties.I

@@ -679,19 +679,18 @@ clear_z_order() {
  * mouse can move outside the window and the mouse coordinates are relative to
  * its position in the window.
  *
- * M_relative (OSX or Unix/X11 only): a mode where only relative movements are
- * reported; particularly useful for FPS-style mouse movements where you have
- * hidden the mouse pointer and are are more interested in how fast the mouse
- * is moving, rather than precisely where the pointer is hovering.
- *
- * This has no effect on Windows.  On Unix/X11, this requires the Xxf86dga
- * extension to be available.
- *
  * M_confined: this mode reports absolute mouse positions, but confines the
- * mouse pointer to the window boundary.  It can portably replace M_relative
- * for an FPS, but you need to periodically move the pointer to the center of
- * the window and track movement deltas.
+ * mouse pointer to the window boundary.  The reported mouse positions will
+ * never be outside of the window boundary.
  *
+ * M_relative: a mode where only relative movements are reported; particularly
+ * useful for FPS-style mouse movements where you have hidden the mouse
+ * pointer and are are more interested in how fast the mouse is moving, rather
+ * than precisely where the pointer is hovering.  The reported positions still
+ * appear to be absolute, but they can go toward negative or positive infinity
+ * without being constrained by the window (or screen) dimensions.  Since the
+ * position of the mouse cursor becomes meaningless in this mode, it is
+ * recommended to combine this with the cursor_hidden flag.
  */
 INLINE void WindowProperties::
 set_mouse_mode(MouseMode mode) {