pgsql: Rename jsonb_hash_ops to jsonb_path_ops.

Rename jsonb_hash_ops to jsonb_path_ops.

There's no longer much pressure to switch the default GIN opclass for
jsonb, but there was still some unhappiness with the name "jsonb_hash_ops",
since hashing is no longer a distinguishing property of that opclass,
and anyway it seems like a relatively minor detail. At the suggestion of
Heikki Linnakangas, we'll use "jsonb_path_ops" instead; that captures the
important characteristic that each index entry depends on the entire path
from the document root to the indexed value.

Also add a user-facing explanation of the implementation properties of
these two opclasses.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/12e611d43e6efbf0e36014a3055ed47366facee7

Modified Files
--------------
doc/src/sgml/gin.sgml | 4 +--
doc/src/sgml/json.sgml | 45 +++++++++++++++++++++++++++++----
src/backend/utils/adt/jsonb_gin.c | 27 ++++++++++----------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_amop.h | 6 ++---
src/include/catalog/pg_opclass.h | 2 +-
src/include/catalog/pg_opfamily.h | 4 +--
src/include/catalog/pg_proc.h | 8 +++---
src/include/utils/jsonb.h | 12 ++++-----
src/test/regress/expected/jsonb.out | 4 +--
src/test/regress/expected/jsonb_1.out | 4 +--
src/test/regress/sql/jsonb.sql | 4 +--
12 files changed, 78 insertions(+), 44 deletions(-)