Update index.mdx (#727)

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Author: mibali

docs/admin/auth/index.mdx

@@ -334,6 +334,7 @@ Then add the following lines to your [site configuration](/admin/config/site_con
         "displayName": "Bitbucket Server",
         "clientID": "replace-with-the-oauth-client-id",
         "clientSecret": "replace-with-the-oauth-client-secret"
+        "allowSignup": false // This is set to false by default, which means that any bitbucket server user cannot automatically sign up to access your instance.
       }
     ]
 ```