Refine Precipitation's DateTime method comment

The Precipitation's DateTime method comment was adjusted to better reflect its functionality. Instead of stating it returns true if the precipitation data was available at the time of the query or not, it was clarified that the method actually returns the DateTime when the Precipitation value was recorded.

precipitation.go

@@ -20,8 +20,7 @@ func (p Precipitation) IsAvailable() bool {
 	return !p.na
 }
 
-// DateTime returns true if an Precipitation value was
-// available at time of query
+// DateTime returns the DateTime when the Precipitation value was recorded
 func (p Precipitation) DateTime() time.Time {
 	return p.dt
 }