tools: fix json doc generation

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

PR-URL: nodejs#5943
Fixes: nodejs#5942
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>

tools/doc/json.js

@@ -183,7 +183,7 @@ function processList(section) {
   list.forEach(function(tok) {
     var type = tok.type;
     if (type === 'space') return;
-    if (type === 'list_item_start') {
+    if (type === 'list_item_start' || type === 'loose_item_start') {
       var n = {};
       if (!current) {
         values.push(n);