pgsql: Minor code review for json.c.

Minor code review for json.c.

Improve commenting, conform to project style for use of ++ etc.
No functional changes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f871ef74a5560377d541e6d94704f30bcbcdb779

Modified Files
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src/backend/utils/adt/json.c |  164 +++++++++++++++++++++++-------------------
1 files changed, 91 insertions(+), 73 deletions(-)