Minor grammar cleanup

Thanks for the reminder: https://github.com/hsutter/cppfront/pull/726/files#r1404469347

Author: hsutter

source/parse.h

@@ -7633,12 +7633,9 @@ class parser
     //G
     //G return-list:
     //G     expression-statement
-    //G     '->' return-direction? type-id
+    //G     '->' parameter-direction? type-id
     //G     '->' parameter-declaration-list
     //G
-    //G return-direction: one of
-    //G     'forward' 'move'
-    //G
     //G contract-seq:
     //G     contract
     //G     contract-seq contract