SQL/JSON: Various improvements to SQL/JSON query function docs

SQL/JSON: Various improvements to SQL/JSON query function docs

1. Remove the keyword SELECT from the examples to be consistent
with the examples of other JSON-related functions listed on the
same page.

2. Add <synopsis> tags around the functions' syntax definition

3. Capitalize function names in the syntax synopsis and the examples

4. Use <itemizedlist> lists for dividing the descriptions of
   individual functions into bullet points

5. Significantly rewrite the description of wrapper clauses of
   JSON_QUERY

6. Significantly rewrite the descriptions of ON ERROR / EMPTY
   clauses of JSON_QUERY() and JSON_VALUE() functions

7. Add a note about how JSON_VALUE() and JSON_QUERY() differ when
   returning a JSON null result

8. Move the description of the PASSING clause from the descriptions
   of individual functions into the top paragraph

And other miscellaneous text improvements, typo fixes.

Suggested-by: Thom Brown <thom@linux.com>
Suggested-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Erik Rijkers <er@xs4all.nl>
Discussion: https://postgr.es/m/CAA-aLv7Dfy9BMrhUZ1skcg=OdqysWKzObS7XiDXdotJNF0E44Q@mail.gmail.com
Discussion: https://postgr.es/m/CAKFQuwZNxNHuPk44zDF7z8qZec1Aof10aA9tWvBU5CMhEKEd8A@mail.gmail.com