docs: Make it clear that the use of uri is specified by saml2int.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2571 44740490-163a-0410-bde0-09ae8108e29a

docs/simplesamlphp-idp.txt

@@ -150,7 +150,8 @@ files, see the [IdP hosted reference](simplesamlphp-reference-idp-hosted).
 Using the `uri` NameFormat on attributes
 ----------------------------------------
 
-It is generally recommended to use the `urn:oasis:names:tc:SAML:2.0:attrname-format:uri` attribute NameFormat in new installations.
+The [interoperable SAML 2 profile](http://saml2int.org/profile/current) specifies that attributes should be delivered using the `urn:oasis:names:tc:SAML:2.0:attrname-format:uri` NameFormat.
+We therefore recommended enabling this in new installations.
 This can be done by adding the following to the saml20-idp-hosted configuration:
 
     'AttributeNameFormat' => 'urn:oasis:names:tc:SAML:2.0:attrname-format:uri',