Some updates on IRC.md

http://help.twitch.tv/customer/portal/articles/1302780-twitch-irc

I updated some things that were missing/overlooked to more accurately reflect the above help article.

The more important things:
- channelname.jtvirc.com is not used for ping/pong messages
- I think the command/message limit is relevant/important so I added that
- the restriction on joining multiple channels does not apply anymore

Hope I did this right :)

Edit: Oh, and the second commit was because I missed the generic channelname in the JOIN section in my first one