SQL/JSON: documentation - Mailing list pgsql-hackers

Attached patch with draft of SQL/JSON documentation written by
Liudmila Mantrova, Oleg Bartunov and me.

Also it can be found in our sqljson repository on sqljson_doc branch:
https://github.com/postgrespro/sqljson/tree/sqljson_doc

We continue to work on it.

-- 
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company