add docs for channel.off function (#3629)
add docs for channel.off function (#3629)


phoenixframework/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,jwarwick/phoenix,eksperimental/phoenix,phoenixframework/phoenix,0x00evil/phoenix,michalmuskala/phoenix,rafbgarcia/phoenix,keathley/phoenix,optikfluffel/phoenix,eksperimental/phoenix,keathley/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,optikfluffel/phoenix,nshafer/phoenix,michalmuskala/phoenix,nshafer/phoenix,0x00evil/phoenix,jwarwick/phoenix,nshafer/phoenix