Add ALLOW_ONLY_EXTERNAL_REGISTRATION to config cheat sheet (#8986)

ALLOW_ONLY_EXTERNAL_REGISTRATION was missed off the cheat sheet. This PR adds it.

Fix #8180

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -306,6 +306,7 @@ relation to port exhaustion.
 - `AUTO_WATCH_ON_CHANGES`: **false**: Enable this to make users watch a repository after their first commit to it
 - `DEFAULT_ORG_VISIBILITY`: **public**: Set default visibility mode for organisations, either "public", "limited" or "private".
 - `DEFAULT_ORG_MEMBER_VISIBLE`: **false** True will make the membership of the users visible when added to the organisation.
+- `ALLOW_ONLY_EXTERNAL_REGISTRATION`: **false** Set to true to force registration only using third-party services.
 
 ## Webhook (`webhook`)