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