rest_api_authentication.md: anon. user/login needed

Author: adriendupuis

docs/api/rest_api/rest_api_authentication.md

@@ -21,7 +21,7 @@ For other security related subjects, see:
 !!! caution "SiteAccess login"
 
     The anonymous user is used to access the ressource even if authentification with the REST server has been established.
-    Anonymous user must have user/login policy on the SiteAccess used by the REST domain.
+    Anonymous user must have `user/login` policy on the SiteAccess matched by the REST domain or passed through [`X-Siteaccess` header](rest_requests.md#siteaccess).
 
 ## Session-based authentication