Add a user guide on payments

src/pretix/base/payment.py

@@ -150,9 +150,9 @@ class BasePaymentProvider:
              forms.BooleanField(
                  label=_('Calculate the fee from the total value including the fee.'),
                  help_text=_('We recommend you to enable this if you want your users to pay the payment fees of your '
-                             'payment provider. <a href="/control/help/payment/fee_reverse" target="_blank">Click here '
+                             'payment provider. <a href="{docs_url}" target="_blank">Click here '
                              'for detailled information on what this does.</a> Don\'t forget to set the correct fees '
-                             'above!'),
+                             'above!').format(docs_url='https://docs.pretix.eu/en/latest/user/payments/fees.html'),
                  required=False
              )),
             ('_invoice_text',