The attached zip file contains "json.xml" and "images/json.ebnf" (for an overview over the syntax).
Unfortunately, not having any experiences with the used DTD, I was not able to define an external link (to the JSON RFC); gave the link verbatimly, please check out the comment containing "TODO".
Unzip in the directory:
oorexx\docs\trunk\rexxextensions\en-US
Anonymous
JSON documentation has already been added for 5.0