data tree DOC added a note about diff and defaults behavior
Refs #1445
diff --git a/src/tree_data.h b/src/tree_data.h
index c90add1..c1ebd15 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -1615,6 +1615,11 @@
/**
* @brief Apply the whole diff on a data tree but restrict the operation to one module.
*
+ * __!! Caution !!__
+ * If applying a diff that was created __without__ the ::LYD_DIFF_DEFAULTS flag, there may be some duplicate values
+ * created. Unless the resulting tree is validated (and default values thus consolidated), using it further
+ * (such as applying another diff) may cause unexpected results or errors.
+ *
* @param[in,out] data Data to apply the diff on.
* @param[in] diff Diff to apply.
* @param[in] mod Module, whose diff/data only to consider, NULL for all modules.