drm/amdgpu: improve GART and GTT documentation

Document difference between amdgpu.gartsize and amdgpu.gttsize
module parameters, as initially explained by Alex Deucher here:
https://lists.freedesktop.org/archives/dri-devel/2022-October/375358.html



v2: minor cleanups (Alex)

Signed-off-by: Peter Maucher <bellosilicio@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>