docs(nav): add docstrings

nav.py

@@ -35,16 +35,19 @@ LAUNCH_FUEL = 380.0  # tonnes of fuel to escape Earth's gravity
 
 
 def distance(a, b):
+    """Return the distance between two neighboring waypoints."""
     if (a, b) in DISTANCES:
         return DISTANCES[(a, b)]
     return DISTANCES[(b, a)]
 
 
 def legs(route):
+    """Return the legs (pairs of consecutive waypoints) of a route."""
     return [(route[i], route[i + 1]) for i in range(len(route) - 1)]
 
 
 def plan(destination):
+    """Return route, distance, travel time (days) and fuel (t)."""
     route = ROUTES[destination]
     total = 0.0
     fuel = 0.0