drm/amdgpu: Expand kernel-doc in amdgpu_ring

drm/amdgpu: Expand kernel-doc in amdgpu_ring

Expand the kernel-doc about amdgpu_ring and add some tiny improvements.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>