Update migration guide for acl

doc/migrations/5.0.0_5.1.0.md

@@ -2,6 +2,11 @@
 
 The configuration format has slightly changed and you might need to amend `mongooseim.toml`.
 
+### Section `acl`
+
+The implicit check for user's domain in patterns is now configurable and the default behaviour (previously undocumented) is more consistent - the check is always performed unless disabled with `match = "all"`.
+See the description of [`current_domain`](../configuration/acl.md#aclmatch) for more details.
+
 ### Section `auth`
 
 * Each authentication method needs a TOML section, e.g. if you have the `rdbms` method enabled, you need to have the `[auth.rdbms]` section in the configuration file, even if it is empty. The `methods` option is not required anymore and especially if you are using only one method, you can remove it.