avutil/tree: improve documentation for av_tree_find, av_tree_insert

This documents the additional constness, and provides a useful libc
reference for the API specification of the comparator.
Reviewed-by: Henrik Gramner <henrik@gramner.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavutil/tree.h

@@ -56,6 +56,8 @@ struct AVTreeNode *av_tree_node_alloc(void);
  * @param next If next is not NULL, then next[0] will contain the previous
  *             element and next[1] the next element. If either does not exist,
  *             then the corresponding entry in next is unchanged.
+ * @param cmp compare function used to compare elements in the tree,
+ *            API identical to that of Standard C's qsort
  * @return An element with cmp(key, elem) == 0 or NULL if no such element
  *         exists in the tree.
  */
@@ -99,7 +101,8 @@ void *av_tree_find(const struct AVTreeNode *root, void *key,
  *                 return av_tree_insert(rootp, key, cmp, next);
  *             }
  *             @endcode
- * @param cmp compare function used to compare elements in the tree
+ * @param cmp compare function used to compare elements in the tree, API identical
+ *            to that of Standard C's qsort
  * @return If no insertion happened, the found element; if an insertion or
  *         removal happened, then either key or NULL will be returned.
  *         Which one it is depends on the tree state and the implementation. You