Add comment deprecating the Documentation's get_subject-method

& clearify why and what to use instead.
= there is no need for the get_subject-method!

aleksis/apps/alsijil/models.py

@@ -101,6 +101,9 @@ class Documentation(CalendarEvent):
         blank=True, null=True, verbose_name=_("Participation touched at")
     )
 
+    # get_subject is deprecated
+    # Use doc.subject, .course & .teachers directly instead.
+    # The amends logic is done onetime for each documentation in the create_from_lesson_event method.
     def get_subject(self) -> str:
         if self.subject:
             return self.subject