docs: documented new Signpost functionality

Co-authored-by: Allison Ryan <77211884+allisonryan0002@users.noreply.github.com>

packages/pinball_components/lib/src/components/signpost.dart

@@ -41,6 +41,8 @@ extension on SignpostSpriteState {
 
 /// {@template signpost}
 /// A sign, found in the Flutter Forest.
+///
+/// Lights up a new sign whenever all three [DashNestBumper]s are hit.
 /// {@endtemplate}
 class Signpost extends BodyComponent with InitialPosition {
   /// {@macro signpost}