Invoicing: Allow plugins to add data (#5452)

* Allow plugins to add data to invoices

* Add documentation

src/pretix/base/signals.py

@@ -596,6 +596,18 @@ multiple events. Receivers should return a subclass of pretix.base.exporter.Base
 The ``sender`` keyword argument will contain an organizer.
 """
 
+build_invoice_data = EventPluginSignal()
+"""
+Arguments: ``invoice``
+
+This signal is sent out every time an invoice is built, after the invoice model was created
+and filled and before the PDF generation task is started. You can use this to make changes
+to the invoice, but we recommend to mostly use it to add content to ``Invoice.plugin_data``.
+You are responsible for saving any changes to the database.
+
+As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
+"""
+
 validate_order = EventPluginSignal()
 """
 Arguments: ``payments``, ``positions``, ``email``, ``locale``, ``invoice_address``,