[Docs] Add hints on how to edit substitutions

docs/admin/10_managing_data.rst

@@ -2,8 +2,12 @@ Managing timetable and substitution data
 ========================================
 
 Currently, CHronos does not provide an interface for
-interactively managing timetable and subsitution data.
+interactively managing timetable data.
 
 Instead, data is imported from an external source. The
 official distribution handbook contains documentation about
 the existing integration apps.
+
+While there is basic support for editing substitutions, the
+interface should not be used when substitution data is
+imported from an external source.

docs/user/15_manage_substitutions.rst

+Managing substitutions
+======================
+
+AlekSIS has basic support for managing substitutions and cancellations.
+
+In the mnue under `Timetable → Daily lessons`, a list of all lessons taking
+place on a day can be found.
+
+.. image:: ../_static/daily_lessons.png
+  :width: 600
+  :alt: Daily lessons
+
+The list allows editing a substitution for each lesson.
+
+.. image:: ../_static/substitution_edit.png
+  :width: 600
+  :alt: Edit a substitution
+
+When editing a substitution, the week, date, and lesson is pre-filled.
+The fields for `teacher`, `room`, and `subject` allow replacing
+the respective information for the selected lesson.
+
+It should be noted that support for editing substitutions is currently
+very basic, and the feature should not be used when substitution data
+is imported from an external source.