Fix a couple of outdated comments now that we have MERGE RETURNING

This has been supported since c649fa2. Discussion: https://postgr.es/m/CAApHDvpqp6vtUzG-_josUEiBGyqnrnVxJ-VdF+hJLXjHdHzsyQ@mail.gmail.com

Author: david-rowley

src/include/nodes/plannodes.h

@@ -53,9 +53,9 @@ typedef struct PlannedStmt
 
 uint64queryId;/* query identifier (copied from Query) */
 
-boolhasReturning;/* is it insert|update|delete RETURNING? */
+boolhasReturning;/* is it insert|update|delete|merge RETURNING? */
 
-boolhasModifyingCTE;/* has insert|update|delete in WITH? */
+boolhasModifyingCTE;/* has insert|update|delete|merge in WITH? */
 
 boolcanSetTag;/* do I set the command result tag? */