Deprecate #<mask> and clarify $<mask>

[jwheare]

https://datatracker.ietf.org/doc/html/rfc1459#section-4.4.1
https://datatracker.ietf.org/doc/html/rfc2812#section-3.3.1

Opers can send messages to e.g. $* or $$* or $*.example.com for global notices, and they look like this for the recipient

:cool_oper_guy!user@host NOTICE $* :[Global notice] Shutting down some servers, enjoy the netsplits

Should probably tell clients to handle targets starting with $ specifically. Some clients treat this $* target as its own buffer, (especially if the cool oper used PRIVMSG instead of NOTICE by mistake) some send them to the PM buffer with cool_oper_guy, but they should probably just go to the server console/buffer.

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There also used to be a #<mask> target as well, but it's not implemented anywhere (presumably cos clients treated them as channels to hilarious effect) so should probably document that as dead.