I pondered mentioning the 2s interval but the environment variable itself doesn't have a default, it's either set or unset which determines whether it provides the default or not. IMO, adding this detail produces too much text for little benefit.
On second thought, something like:
WATCH_INTERVAL: an integer number of seconds; overrides the default two second \watch interval
(not quite happy but it's close...keeps the 2s reference point)
