The current json doctool does not support grouped optional params

[firedfox]

• Version: master 54a5287
• Platform: Darwin firedfox-mbp.local 15.4.0 Darwin Kernel Version 15.4.0: Fri Feb 26 22:08:05 PST 2016; root:xnu-3248.40.184~3/RELEASE_X86_64 x86_64
• Subsystem: tools, doc

Some optional params in the doc are like:
methodName([param0, ][param1]) or methodName([param0[, param1]])
Such as:

### socket.send(msg, [offset, length,] port, address[, callback])
## fs.writeSync(fd, data[, position[, encoding]])

The current json doctool does not support grouped brackets yet. It fails to figure out what params are optional.

I'll make a PR to fix it.