drm: Update TTM initialization documentation

ttm_global_reference was renamed to drm_global_reference.  This updates
the documentation to reflect that.  While we are there, document the
drm_global_reference API and update the initialization interface
documentation.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
[danvet: Keep the warning, ttm docs are still massively inadequate.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161228143216.26821-7-krisman@collabora.co.uk

drivers/gpu/drm/drm_global.c

@@ -63,6 +63,18 @@ void drm_global_release(void)
 	}
 }
 
+/**
+ * drm_global_item_ref - Initialize and acquire reference to memory
+ * object
+ * @ref: Object for initialization
+ *
+ * This initializes a memory object, allocating memory and calling the
+ * .init() hook. Further calls will increase the reference count for
+ * that item.
+ *
+ * Returns:
+ * Zero on success, non-zero otherwise.
+ */
 int drm_global_item_ref(struct drm_global_reference *ref)
 {
 	int ret = 0;
@@ -97,6 +109,17 @@ error_unlock:
 }
 EXPORT_SYMBOL(drm_global_item_ref);
 
+/**
+ * drm_global_item_unref - Drop reference to memory
+ * object
+ * @ref: Object being removed
+ *
+ * Drop a reference to the memory object and eventually call the
+ * release() hook.  The allocated object should be dropped in the
+ * release() hook or before calling this function
+ *
+ */
+
 void drm_global_item_unref(struct drm_global_reference *ref)
 {
 	struct drm_global_item *item = &glob[ref->global_type];