Add comment for pgtype.Interval struct

Comment explicitly states that pgtype.Interval is a nullable
postgres type.

pgtype/interval.go

@@ -26,6 +26,21 @@ type IntervalValuer interface {
 	IntervalValue() (Interval, error)
 }
 
+// Interval represents an interval that may be null.
+// Interval implements the [Scanner] interface so
+// it can be used as a scan destination:
+//
+//	var i Interval
+//	err := conn.QueryRow(context.Background(), "SELECT elapsed_time FROM foo WHERE id=$1", id).Scan(&i)
+//	...
+//	if i.Valid {
+//	   // use i.Days...
+//	} else {
+//	   // NULL value
+//	}
+//
+// When using as a parameter for prepared statement
+// the [Valid] field has to be explicitly set to [true] by the user.
 type Interval struct {
 	Microseconds int64
 	Days         int32