DocBook/drm: Improve flow of GPU/CPU coherence sentence

	provides memory to back each object.  When mapped into the GTT

	or used in a command buffer, the backing pages for an object are

	flushed to memory and marked write combined so as to be coherent

	with the GPU.  Likewise, when the GPU finishes rendering to an object,

	if the CPU accesses it, it must be made coherent with the CPU's view

	with the GPU.  Likewise, if the CPU accesses an object after the GPU

	has finished rendering to the object, then the object must be made

	coherent with the CPU's view

	of memory, usually involving GPU cache flushing of various kinds.

	This core CPU<->GPU coherency management is provided by the GEM

	set domain function, which evaluates an object's current domain and