Add documentation to password_providers config option (#7238)

docs/password_auth_providers.md

@@ -9,7 +9,10 @@ into Synapse, and provides a number of methods by which it can integrate
 with the authentication system.
 
 This document serves as a reference for those looking to implement their
-own password auth providers.
+own password auth providers. Additionally, here is a list of known
+password auth provider module implementations:
+
+* [matrix-synapse-ldap3](https://github.com/matrix-org/matrix-synapse-ldap3/)
 
 ## Required methods
 

docs/sample_config.yaml

@@ -1657,7 +1657,19 @@ email:
   #template_dir: "res/templates"
 
 
-#password_providers:
+# Password providers allow homeserver administrators to integrate
+# their Synapse installation with existing authentication methods
+# ex. LDAP, external tokens, etc.
+#
+# For more information and known implementations, please see
+# https://github.com/matrix-org/synapse/blob/master/docs/password_auth_providers.md
+#
+# Note: instances wishing to use SAML or CAS authentication should
+# instead use the `saml2_config` or `cas_config` options,
+# respectively.
+#
+password_providers:
+#    # Example config for an LDAP auth provider
 #    - module: "ldap_auth_provider.LdapAuthProvider"
 #      config:
 #        enabled: true