Rephrase a few comments for clarity.

I was confused by what "intended to be parallel serially" meant, until
Robert Haas and David G. Johnston explained it. Rephrase the comment to
make it more clear, using David's suggested wording.

Discussion: https://www.postgresql.org/message-id/1fec9022-41e8-e484-70ce-2179b08c2092%40iki.fi