avutil/tree: Document the guaranteed ordering of compare arguments for av_tree_find()

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/tree.h

@@ -58,6 +58,9 @@ struct AVTreeNode *av_tree_node_alloc(void);
  *             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
+ *            It is guranteed that the first and only the first argument to cmp()
+ *            will be the key parameter to av_tree_find(), thus it could if the
+ *            user wants, be a different type (like an opaque context).
  * @return An element with cmp(key, elem) == 0 or NULL if no such element
  *         exists in the tree.
  */