drm/ttm: add kerneldoc for enum ttm_caching

Briefly describe what this is all about.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908132933.3269-4-christian.koenig@amd.com