Update the list of mode prefixes and document them
Update the list of mode prefixes and document them

RFC 2811 mentions only @ and +. However, IRC daemons have extended it
using the PREFIX parameter to use many more prefixes:
http://tools.ietf.org/html/draft-brocklesby-irc-isupport-03

Fixes: https://bugs.freedesktop.org/30635
Signed-off-by: Will Thompson <5822fdda638242a44ccfa68f146ccf6f257c5cbd@collabora.co.uk>

freedesktop-unofficial-mirror/telepathy__telepathy-idle,freedesktop-unofficial-mirror/telepathy__telepathy-idle,freedesktop-unofficial-mirror/telepathy__telepathy-idle