pgsql: SQL/JSON: Miscellaneous fixes and improvements

SQL/JSON: Miscellaneous fixes and improvements

This addresses some post-commit review comments for commits 6185c973,
de3600452, and 9425c596a0, with the following changes:

* Fix JSON_TABLE() syntax documentation to use the term
"path_expression" for JSON path expressions instead of
"json_path_specification" to be consistent with the other SQL/JSON
functions.

* Fix a typo in the example code in JSON_TABLE() documentation.

* Rewrite some newly added comments in jsonpath.h.

* In JsonPathQuery(), add missing cast to int before printing an enum
value.

Reported-by: Jian He <jian(dot)universality(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxG_e0QLCgaELrr2ZNz7AxPeGCNKAORe3fHtFCQLsH4J4Q@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ef744ebb7340d42361ebd88d068b27b17e69de18

Modified Files
--------------
doc/src/sgml/func.sgml | 9 +++++----
src/backend/utils/adt/jsonpath_exec.c | 2 +-
src/include/utils/jsonpath.h | 8 +++-----
3 files changed, 9 insertions(+), 10 deletions(-)