Fix and polish JSON_TABLE documentation

Fix and polish JSON_TABLE documentation

Correct the JSON_TABLE synopsis to place the ON ERROR clause after the
PLAN clause, matching the grammar (the previous order could not be
typed).  Replace a dangling reference to json_path_specification with
path_expression, the term the synopsis actually defines.  Mark up the
PLAN DEFAULT keywords with <literal> and fix a couple of wording issues.
Also list OUTER before INNER consistently, including in the parent/child
join description, where OUTER is the default.

Author: Thom Brown <thom@linux.com>
Discussion: https://postgr.es/m/CAA-aLv5PGAFmAEpbhKQz8wppoOOTHfo-=LJb2sAB3974-9QtOw@mail.gmail.com