Queue password checks and shorten busy retries (Refs #95)

docs/guides/authentication.md

@@ -76,6 +76,11 @@ Login, initial setup and password verification are rate-limited with persistent
 changes and successful management requests without recording passwords, tokens
 or request bodies. Administrators can review recent entries in Users.
 
+The password service runs at most two scrypt operations at once and queues up to
+16 more for up to two seconds. If that queue fills or the wait expires, the
+request returns HTTP 429 with `Retry-After: 2`. The persistent rate limits still
+return `Retry-After: 900` when an account or address reaches its attempt limit.
+
 An administrator can reset another user's password from Users. If the only
 administrator forgets their password, an operator with local controller/container
 access can run the recovery command in an interactive terminal: