SQL documentation doesn't list json functions

It seems comdb2 now supports sqlite's json functions (comdb2 version from internal unstable dpkg distribution):
```
cdb2sql> SELECT json_extract('{"a":2,"c":[4,5,{"f":7}]}', '$.c[2].f')
(json_extract('{"a":2,"c":[4,5,{"f":7}]}', '$.c[2].f')=7)
```

But these are not documented on:
https://bloomberg.github.io/comdb2/sql.html

Start with the SQL documentation at https://bloomberg.github.io/comdb2/sql.html and compare its documented functions with the SQLite JSON functions shown in the issue's cdb2sql example. Confirm the supported JSON functions in comdb2, then update the SQL documentation so those functions are listed.

Written by the indexing model from the issue text.