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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8b1d2685b82fd0ab09c2ba36081e53248c3b09cb

Modified Files
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src/backend/executor/nodeSeqscan.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)