As of commit e5253fdc, it is no longer true that the leader always
executes the subplan of a Gather Merge node.  Remove comment to that
effect.

Back-patch to 11.

Discussion: https://postgr.es/m/CA%2BhUKGJEaZJYezXAOutuiWT%2BfxCA44%2BoKtVPAND2ubLiigR%3D-w%40mail.gmail.com