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 `only works` and `Useful with lesser moderation`. At least freenode.net calls `+z` as lesser moderation.