tools,doc: fix json for grouped optional params

Current tools/doc/json.js only supports one bracket style for optional
params methodName(param0[,param1],param2). Add support to other styles
such as methodName(param0,[param1,]param2) or
methodName(param0[,param1,param2]) or
methodName(param0[,param1[,param2]]).

PR-URL: https://github.com/nodejs/node/pull/5977
Fixes: https://github.com/nodejs/node/issues/5976


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>