tools: fix json doc generation

Current processList function in tools/doc/json.js does not recognise
{"type":"loose_item_start"}. Fix it.

PR-URL: https://github.com/nodejs/node/pull/5943
Fixes: https://github.com/nodejs/node/issues/5942


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>