drm/ttm: add some general module kerneldoc

For now just a brief description of what TTM is all about.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908132933.3269-3-christian.koenig@amd.com