Widget: Allow to pre-fill fields in the invoice address

doc/user/events/widget.rst

@@ -183,6 +183,8 @@ with that information::
     <pretix-widget event="https://pretix.eu/demo/democon/"
         data-attendee-name-given-name="John"
         data-attendee-name-family-name="Doe"
+        data-invoice-address-name-given-name="John"
+        data-invoice-address-name-family-name="Doe"
         data-email="test@example.org"
         data-question-L9G8NG9M="Foobar">
     </pretix-widget>
@@ -200,6 +202,11 @@ This works for the pretix Button as well. Currently, the following attributes ar
   ``data-attendee-name-latin-transcription``. If you don't know or don't care, you can also just pass a string as
   ``data-attendee-name``, which will pre-fill the last part of the name, whatever that is.
 
+* ``data-invoice-address-FIELD`` will  pre-fill the corresponding field of the invoice address. Possible values for
+  ``FIELD`` are ``company``, ``street``, ``zipcode``, ``city`` and ``country``, as well as fields specified by the
+  naming scheme such as ``name-title`` or ``name-given-name`` (see above). ``country`` expects a two-character
+  country code.
+
 Any configured pretix plugins might understand more data fields. For example, if the appropriate plugins on pretix
 Hosted or pretix Enterprise are active, you can pass the following fields: