Clarification about NET_SYNC_REQUEST

[Soeren Sandmann <sandmann daimi au dk>]

I committed the attached patch. It makes it clear that the window
manager must not send a sync request of 0. I can't access the kwin
patch anymore, but I am fairly sure that it didn't send 0 requests.

The gtk+ patch relies on this behavior because it uses 0 as a 'no
response needed' value. 


Søren