#411 automatically add "<channel> is only necessary..."

[Valentin Lorentz]

Hello,

Lot of commands has a docstring ending with : "<channel> is only necessary if the message isn't sent in the channel itself.".
This commands can be reconized, because they use the 'channel' converter.</channel>

Why not adding automatically this sentence to the help of every command which use 'channel', instead of repeating it ?

Best regards,
ProgVal