auth/sql: Add section about using worker processes

Author: sirainen

data/updates.js

@@ -25,6 +25,7 @@ export const updates = {
 	auth_policy_reject: '2.4.0',
 	auth_protocol_handshake_changed: '2.4.0',
 	auth_server_common_secured: '2.4.0',
+	auth_sql_workers_changed: '2.4.1',
 	dcrypt_same_cipher_algo_added: '2.4.0',
 	dict_protocol_v4: '2.4.0',
 	doveadm_ex_expired_code: '2.4.0',

docs/core/config/auth/databases/sql.md

@@ -130,6 +130,15 @@ fields and use [[link,auth_prefetch]] to use those fields.
 If you're using Dovecot's deliver you'll still need to have the
 [[setting,userdb_sql_query]] working.
 
+## Worker processes
+
+MySQL and SQLite lookups are always done using `auth-worker` processes.
+
+[[changed,auth_sql_workers_changed]] PostgreSQL and Cassandra lookups are
+done by default in the main `auth` process. They can be made to use
+`auth-worker` processes by setting [[setting,passdb_use_worker,yes]] and
+[[setting,userdb_use_worker,yes]].
+
 ## High Availability
 
 You can add multiple [[link,sql_mysql]] or [[link,sql_postgresql]] settings to