Documentation/gpu: Update documentation on drm-shared-*

Clarify the documentation in preparation for updated
helpers which check the handle count as well as whether
a dma-buf has been attached.

Link: https://lore.kernel.org/all/20231207180225.439482-1-alexander.deucher@amd.com/
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.keonig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>