Add login_url in section 3.2 ("The config file") (#710)

* Add login_url in section 3.2 ("The config file")

* Add account_label

Author: cbiesinger

spec/index.bs

@@ -1881,8 +1881,14 @@ The {{IdentityProviderAPIConfig}} object's members have the following semantics:
     :   <dfn>disconnect_endpoint</dfn>
     ::  A URL that points to an HTTP API that complies with the [[#idp-api-disconnect-endpoint]]
         API.
+    :   <dfn>login_url</dfn>
+    ::  A URL that can be used to log in to this IdP. This is a user-facing URL that can be shown in
+        a popup window.
     :   <dfn>branding</dfn>
     ::  A set of {{IdentityProviderBranding}} options.
+    :   <dfn>account_label</dfn>
+    ::  An optional string that needs to match the {{IdentityProviderAccount/label_hints}} provided
+        by the IdP.
 </dl>
 
 The {{IdentityProviderBranding}} enables an [=IDP=] to express their branding