docs - add description for jsonb data type
description is based on a comment for the jsonb_recv function
that I found in gpdb/src/backend/utils/adt/jsonb.c
<row>
<entry colname="col1">jsonb</entry>
<entry colname="col2"/>
- <entry colname="col3">???</entry>
- <entry colname="col4">???</entry>
- <entry colname="col5">???</entry>
+ <entry colname="col3">1 byte + binary string</entry>
+ <entry colname="col4">json of any length in a decomposed binary format</entry>
+ <entry colname="col5">variable unlimited length</entry>
</row>
<row>
<entry colname="col1">lseg</entry>
