[Docs] Explain carry-over

docs/user/21_lesson.rst

@@ -28,6 +28,10 @@ Everything entered here should be considered public knowledge.
   :width: 100%
   :alt: Lesson documentation in lesson overview
 
+If enabled in the preferences, lesson documentation is carried over to adjacent
+lessons. So if one subject is held in a double or triple lesson, only one needs
+to be filled in.
+
 Tab 'Personal Notes'
 --------------------
 
@@ -42,6 +46,16 @@ student.
   :width: 100%
   :alt: Personal notes in lesson overview
 
+Behaviour of absences and tardiness
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a student is marked as absent, this information is carried over to all future
+lessons on the same day, meaning that for any teacher holding lessons in the class
+after the one that marked them as absent will automatically see them as absent.
+
+Likewise, if a student returns and is marked as not absent, this is carried over
+to all future lessons.
+
 Tab 'Previous lesson'
 ---------------------