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>

drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c

@@ -231,16 +231,18 @@ module_param_named(vis_vramlimit, amdgpu_vis_vram_limit, int, 0444);
 
 /**
  * DOC: gartsize (uint)
- * Restrict the size of GART in Mib (32, 64, etc.) for testing. The default is -1 (The size depends on asic).
+ * Restrict the size of GART (for kernel use) in Mib (32, 64, etc.) for testing.
+ * The default is -1 (The size depends on asic).
  */
-MODULE_PARM_DESC(gartsize, "Size of GART to setup in megabytes (32, 64, etc., -1=auto)");
+MODULE_PARM_DESC(gartsize, "Size of kernel GART to setup in megabytes (32, 64, etc., -1=auto)");
 module_param_named(gartsize, amdgpu_gart_size, uint, 0600);
 
 /**
  * DOC: gttsize (int)
- * Restrict the size of GTT domain in MiB for testing. The default is -1 (Use 1/2 RAM, minimum value is 3GB).
+ * Restrict the size of GTT domain (for userspace use) in MiB for testing.
+ * The default is -1 (Use 1/2 RAM, minimum value is 3GB).
  */
-MODULE_PARM_DESC(gttsize, "Size of the GTT domain in megabytes (-1 = auto)");
+MODULE_PARM_DESC(gttsize, "Size of the GTT userspace domain in megabytes (-1 = auto)");
 module_param_named(gttsize, amdgpu_gtt_size, int, 0600);
 
 /**