Update docs for improvements in IdP initiated flow

docs/simplesamlphp-idp-more.md

@@ -45,11 +45,16 @@ Here is an example of such a URL:
 
 	https://idp.example.org/simplesaml/saml2/idp/SSOService.php?spentityid=urn:mace:feide.no:someservice
 
-You can also add a RelayState parameter to the IdP-first URL:
+You can also add a `RelayState` parameter to the IdP-first URL:
 
 	https://idp.example.org/simplesaml/saml2/idp/SSOService.php?spentityid=urn:mace:feide.no:someservice&RelayState=https://sp.example.org/somepage
 
-The RelayState parameter is often uset do carry the URL the SP should redirect to after authentication.
+The `RelayState` parameter is often used to carry the URL the SP should redirect to after authentication. It is also possible to specify the Assertion
+Consumer URL with the `ConsumerURL` parameter.
+
+For compatibility with certain SPs, SimpleSAMLphp will also accept the
+`providerId`, `target` and `shire` parameters as aliases for `spentityid`,
+`RelayState` and `ConsumerURL`, respectively.
 
 
 ### IdP first with SAML 1.1