Update pattern-matching.md

Added a note about sealed traits because it is the first usage of them in the guide.

Author: ErikPohl444

_tour/pattern-matching.md

@@ -92,7 +92,7 @@ case class VoiceRecording(contactName: String, link: String) extends Notificatio
 {% endtab %}
 {% endtabs %}
 
-`Notification` is a sealed trait which has three concrete Notification types implemented with case classes `Email`, `SMS`, and `VoiceRecording`. Now we can do pattern matching on these case classes:
+`Notification` is a sealed trait which has three concrete Notification types implemented with case classes `Email`, `SMS`, and `VoiceRecording`. (A sealed trait can be extended only in the same file as its declaration.) Now we can do pattern matching on these case classes:
 
 {% tabs pattern-matching-4 class=tabs-scala-version %}
 {% tab 'Scala 2' for=pattern-matching-4 %}