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
------
master

Details
-------
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(-)