util/list.h: Add docstrings for list_add and list_addtail

Which have easily confused parameters: the first argument is the item to
be added, the second is the list to add to; but this could easily be the
other way around.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14983>

src/util/list.h

@@ -61,6 +61,12 @@ static inline void list_inithead(struct list_head *item)
     item->next = item;
 }
 
+/**
+ * Prepend an item to a list
+ *
+ * @param item The element to add to the list
+ * @param list The list to prepend to
+ */
 static inline void list_add(struct list_head *item, struct list_head *list)
 {
     item->prev = list;
@@ -69,6 +75,12 @@ static inline void list_add(struct list_head *item, struct list_head *list)
     list->next = item;
 }
 
+/**
+ * Append an item to a list
+ *
+ * @param item The element to add to the list
+ * @param list The list to append to
+ */
 static inline void list_addtail(struct list_head *item, struct list_head *list)
 {
     item->next = list;