doc, protocol: Clarify that serial numbers are opaque

Specification does not tell anything about behavior of
serial numbers, except that their values increase. It is
silent about wraparounds, and therefore does not restrict
compositors at all:

- a compositor that returns constant values may claim
  that it increases them by 2**32,
- a compositor that returns decreasing values may claim
  that it increases them by 2**32-1,
- a compositor that returns random values may claim that
  it increases them by 2**32+rand32().

Therefore:
- drop the useless wording about increasing numbers,
- do not specify that wl_display::sync returns the
  event serial number. This serial number cannot be
  passed to any request and hence is useless.

Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>

protocol/wayland.xml

@@ -46,7 +46,7 @@
 	compositor after the callback is fired and as such the client must not
 	attempt to use it after that point.
 
-	The callback_data passed in the callback is the event serial.
+	The callback_data passed in the callback is not specified.
       </description>
       <arg name="callback" type="new_id" interface="wl_callback"
 	   summary="callback object for the sync request"/>