add FAIL * ACCOUNT_REQUIRED to account-registration (#481)

Author: slingamn

extensions/account-registration.md

@@ -177,6 +177,11 @@ or expired.
 Sent by the server if the `REGISTER`/`VERIFY` commands are temporarily
 unavailable.
 
+    FAIL * ACCOUNT_REQUIRED <message>
+
+Sent by the server in response to the end of connection registration (e.g.
+to `USER`, `NICK`, or `CAP END`) if connection registration cannot proceed
+unless the user logs into an account with SASL.
 
 # Examples