Update sample config

docs/sample_config.yaml

@@ -1347,6 +1347,13 @@ saml2_config:
   #
   #grandfathered_mxid_source_attribute: upn
 
+  # Path to a file containing HTML content to serve in case an error happens
+  # when the user gets redirected from the SAML IdP back to Synapse.
+  # If no file is provided, this defaults to some minimalistic HTML telling the
+  # user that something went wrong and they should try authenticating again.
+  #
+  #error_html_path: /path/to/static/content/saml_error.html
+
 
 
 # Enable CAS for registration and login.