data tree BUGFIX change leaf value in its canonical form

Fixes cesnet/netopeer2#143
diff --git a/src/tree_data.h b/src/tree_data.h
index ef5cf82..e5e5347 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -677,7 +677,7 @@
  * @param[in] leaf A leaf node to change.
  * @param[in] val_str String form of the new value to be set to the \p leaf. In case the type is #LY_TYPE_INST
  * or #LY_TYPE_IDENT, JSON node-id format is expected (nodes are prefixed with module names, not XML namespaces).
- * @return 0 on success, non-zero on error.
+ * @return 0 on success, <0 on error, 1 if the (canonical) value matched the original one and no change occured.
  */
 int lyd_change_leaf(struct lyd_node_leaf_list *leaf, const char *val_str);