pgsql: docs: clarify JSONB operator descriptions

docs:  clarify JSONB operator descriptions

No catalog bump as the catalog changes are for SQL operator comments.

Backpatch through 9.5

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2169e878c4a542e41a7d66cbb40d9c6bfde23f57

Modified Files
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doc/src/sgml/func.sgml            |   16 ++++++++++------
doc/src/sgml/json.sgml            |    5 +++--
src/include/catalog/pg_operator.h |    6 +++---
3 files changed, 16 insertions(+), 11 deletions(-)