tree CHANGE RPC's input/output node

save some bytes and add description how it works
diff --git a/src/parser/yin.c b/src/parser/yin.c
index dbb6251..d32b8dd 100644
--- a/src/parser/yin.c
+++ b/src/parser/yin.c
@@ -3907,7 +3907,7 @@
     struct lyxml_elem *sub, *next, root;
     struct lys_node *mnode = NULL;
     struct lys_node *retval;
-    struct ly_mnode_input_output *inout;
+    struct lys_node_rpc_inout *inout;
     int r;
     int c_tpdf = 0;