IgnoreNonVoice: add note regarding IRCds && Fix #210

Not all IRCds implement a `+z` like mode; only ones that do so far iirc are charybdis-based and InspIRCd 2.1 (but that hasn't been ported to the current branch yet).

Fixes #210.
I think this should be either `if the channel is +m` or `if the channel is mode +m` or `if the channel is moderated (+m)`.