Add a "please do not guess" comment

Added a visible comment to remind volunteers to not arbitrarily guess at a location.  Its is better to press "I don't know" than to provide unreliable data.  Examination of the ~20,000 LostAtNight and NightCities taskrun results for actual entered data show significant number of completely wrong "guesses" (100s of miles incorrect) just to get a task completion credit.