Updated models and signals documentation

doc/development/api/general.rst

@@ -31,3 +31,36 @@ will have an attribute ``event``.
 
 ``pretix.control.signals.nav_event``:
     The sidebar navigation when the admin has selected an event.
+
+Order events
+------------
+
+There are multiple signals that will be sent out in the ordering cycle:
+
+``pretix.base.signals.order_placed``:
+    Sent out every time an order has been created. Provides the ``order`` as the only
+    keyword argument.
+
+``pretix.base.signals.order_paid``:
+    Sent out every time an order has been paid. Provides the ``order`` as the only
+    keyword argument.
+
+
+Displaying of log entries
+-------------------------
+
+To display an instance of the ``LogEntry`` model to a human user,
+``pretix.base.signals.logentry_display`` will be sent out with a ``logentry`` argument.
+
+The first received response that is not ``None`` will be used to display the log entry
+to the user.
+
+
+Periodic tasks
+--------------
+
+The ``pretix.base.signals.periodic_task`` is a regular django signal (no pretix event
+signal) that we send out every time the periodic task cronjob runs. This interval
+is not sharply defined, it can be everything between a minute and a day. The actions
+you perform should be idempotent, i.e. it should not make a difference if this is send
+out more often than expected.