drm/ttm: add kerneldoc for enum ttm_caching

Briefly describe what this is all about.

Signed-off-by: Christian König <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/drm-mm.rst

···
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_module.c
    :doc: TTM
 
+.. kernel-doc:: include/drm/ttm/ttm_caching.h
+   :internal:
+
 The Graphics Execution Manager (GEM)
 ====================================
 

include/drm/ttm/ttm_caching.h

···
 
 #define TTM_NUM_CACHING_TYPES	3
 
+/**
+ * enum ttm_caching - CPU caching and BUS snooping behavior.
+ */
 enum ttm_caching {
+	/**
+	 * @ttm_uncached: Most defensive option for device mappings,
+	 * don't even allow write combining.
+	 */
 	ttm_uncached,
+
+	/**
+	 * @ttm_write_combined: Don't cache read accesses, but allow at least
+	 * writes to be combined.
+	 */
 	ttm_write_combined,
+
+	/**
+	 * @ttm_cached: Fully cached like normal system memory, requires that
+	 * devices snoop the CPU cache on accesses.
+	 */
 	ttm_cached
 };