Document RPL_WHOISCHANNELS may always be multi-prefix, and have other characters before channel names

[dwfreed]

Some ircds (solanum, immediate ancestors, and oftc-hybrid definitely, possibly also ircd-hybrid and ratbox by heritage) always do multi-prefix in WHOIS channels listing, regardless of IRCv3 multi-prefix capability. Additionally, these ircds will add an additional character before the channel name if it would not have normally appeared in the list but for an operspy like mode or parameter. Solanum uses ! for this (operspy WHOIS, /whois !nick), and oftc-hybrid uses % (WHOIS while GOD usermode set (+S)).

(Opened an issue instead of a PR because I'm bad with words)