pgsql: Rephrase a few comments for clarity.

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/45fcd758a07fef44a8773202b13f469c9636dffc

Modified Files
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src/backend/executor/nodeIndexonlyscan.c | 4 ++--
src/backend/executor/nodeIndexscan.c     | 4 ++--
src/backend/executor/nodeSeqscan.c       | 4 ++--
3 files changed, 6 insertions(+), 6 deletions(-)