Document the amdgpu buffer object API.

v2: Add a DOC section and some more clarification.
v3: Add some clarification and fix a spelling.
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Samuel Li <Samuel.Li@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>