Update sample config

docs/sample_config.yaml

@@ -1640,11 +1640,11 @@ saml2_config:
   #  - attribute: department
   #    value: "sales"
 
-  # Most metadata XML only contains a single IdP entity. However if the
-  # metadata XML contains multiple IdPs Synapse needs to know which IdP to
-  # redirect users to. `idp_entityid` can be populated with the entity of
-  # the IdP that should be used. For most deployments, this parameter should
-  # be omitted.
+  # If the metadata XML contains multiple IdP entities then the `idp_entityid`
+  # option must be set to the entity to redirect users to.
+  #
+  # Most deployments only have a single IdP entity and so should omit this
+  # option.
   #
   #idp_entityid: 'https://our_idp/entityid'