diff --git a/synapse/config/captcha.py b/synapse/config/captcha.py
index b54dbabbee..7ba0c2de6a 100644
--- a/synapse/config/captcha.py
+++ b/synapse/config/captcha.py
@@ -27,6 +27,7 @@ class CaptchaConfig(Config):
     def default_config(self, **kwargs):
         return """\
         ## Captcha ##
+        # See docs/CAPTCHA_SETUP for full details of configuring this.
 
         # This Home Server's ReCAPTCHA public key.
         recaptcha_public_key: "YOUR_PUBLIC_KEY"