update sample config

docs/sample_config.yaml

@@ -988,6 +988,12 @@ signing_key_path: "CONFDIR/SERVERNAME.signing.key"
 # so it is not normally necessary to specify them unless you need to
 # override them.
 #
+# Once SAML support is enabled, a metadata file will be exposed at
+# https://<server>:<port>/_matrix/saml2/metadata.xml, which you may be able to
+# use to configure your SAML IdP with. Alternatively, you can manually configure
+# the IdP to use an ACS location of
+# https://<server>:<port>/_matrix/saml2/authn_response.
+#
 #saml2_config:
 #  sp_config:
 #    # point this to the IdP's metadata. You can use either a local file or
@@ -998,13 +1004,14 @@ signing_key_path: "CONFDIR/SERVERNAME.signing.key"
 #        - url: https://our_idp/metadata.xml
 #
 #    # By default, the user has to go to our login page first. If you'd like to
-#    # allow IdP-initiated login, set 'allow_unsolicited: True' in an 'sp'
-#    # section:
+#    # allow IdP-initiated login, set 'allow_unsolicited: True' in a
+#    # 'service.sp' section:
 #    #
-#    #sp:
-#    #  allow_unsolicited: True
-#    #
-#    # The rest of sp_config is just used to generate our metadata xml, and you
+#    #service:
+#    #  sp:
+#    #    allow_unsolicited: True
+#
+#    # The examples below are just used to generate our metadata xml, and you
 #    # may well not need it, depending on your setup. Alternatively you
 #    # may need a whole lot more detail - see the pysaml2 docs!
 #