Minor fixes for couple some comments around MERGE RETURNING

I noticed that PlannedStmt.hasReturning and hasModifyingCTE have an
outdated comment now that MERGE supports RETURNING (per commit
c649fa24a)

i.e. these two:

> bool hasReturning; /* is it insert|update|delete RETURNING? */

> bool hasModifyingCTE; /* has insert|update|delete in WITH? */

transformWithClause() has:

/* must be a data-modifying statement */
Assert(IsA(cte->ctequery, InsertStmt) ||
   IsA(cte->ctequery, UpdateStmt) ||
   IsA(cte->ctequery, DeleteStmt) ||
   IsA(cte->ctequery, MergeStmt));

pstate->p_hasModifyingCTE = true;

which eventually makes it into PlannedStmt.hasModifyingCTE.

The attached trivial patch fixes these.

David