Document true/false as supported value.

config/gitlab.yml.example

@@ -288,8 +288,8 @@ production: &base
     # auto_sign_in_with_provider: saml
 
     # CAUTION!
-    # This allows users to login without having a user account first. Define the allowed
-    # providers using an array, e.g. ["saml", "twitter"]
+    # This allows users to login without having a user account first. Define the allowed providers
+    # using an array, e.g. ["saml", "twitter"], or as true/false to allow all providers or none.
     # User accounts will be created automatically when authentication was successful.
     allow_single_sign_on: ["saml"]