Update doc for `get_flash/2` (#3734)
Update doc for `get_flash/2` (#3734)

Specify that `nil` is returned if there is no message for `key`
michalmuskala/phoenix,optikfluffel/phoenix,michalmuskala/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,nshafer/phoenix,eksperimental/phoenix,0x00evil/phoenix,0x00evil/phoenix,nshafer/phoenix,phoenixframework/phoenix,0x00evil/phoenix,phoenixframework/phoenix,nshafer/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,eksperimental/phoenix,jwarwick/phoenix,jwarwick/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix