Expand the GPUVM documentation to better describe the
hardware functionality and use cases it serves.

v2: Fixed a couple of spelling mistakes.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20221201214153.8453-2-alexander.deucher@amd.comSigned-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>