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_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a1dc4ea02044af557e4464029016507ea0940d8c

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(-)