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

Documentation/gpu/drm-mm.rst

@@ -28,7 +28,8 @@ UMA devices.
 The Translation Table Manager (TTM)
 ===================================
 
-TTM design background and information belongs here.
+.. kernel-doc:: drivers/gpu/drm/ttm/ttm_module.c
+   :doc: TTM
 
 The Graphics Execution Manager (GEM)
 ====================================

drivers/gpu/drm/ttm/ttm_module.c

@@ -39,6 +39,18 @@
 
 #include "ttm_module.h"
 
+/**
+ * DOC: TTM
+ *
+ * TTM is a memory manager for accelerator devices with dedicated memory.
+ *
+ * The basic idea is that resources are grouped together in buffer objects of
+ * certain size and TTM handles lifetime, movement and CPU mappings of those
+ * objects.
+ *
+ * TODO: Add more design background and information here.
+ */
+
 /**
  * ttm_prot_from_caching - Modify the page protection according to the
  * ttm cacing mode