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  tags around the functions' syntax definition

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

4. Use  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 
Suggested-by: David G. Johnston 
Reviewed-by: Jian He 
Reviewed-by: Erik Rijkers 
Discussion: https://postgr.es/m/CAA-aLv7Dfy9BMrhUZ1skcg=OdqysWKzObS7XiDXdotJNF0E44Q@mail.gmail.com
Discussion: https://postgr.es/m/CAKFQuwZNxNHuPk44zDF7z8qZec1Aof10aA9tWvBU5CMhEKEd8A@mail.gmail.com