DOC Document fixed issue with non-db summary_fields

en/04_Changelogs/5.1.0.md

@@ -145,6 +145,7 @@ SilverStripe\ORM\DataList:
 - [`DataList::filterAny()`](api:SilverStripe\ORM\DataList::filterAny()) queries on many-many relations that use an aggregate `HAVING` clause now correctly use an `OR` conjunction rather than an incorrect `AND` conjunction.
 - At some point shortly before the release of Silverstripe CMS 4.0.0, SSL support for database connections was accidentally removed. This has now been reinstated - see [Using SSL in database connections](/developer_guides/security/secure_coding#using-ssl-in-database-connections) for more information.
 - The `cascade_duplicates` property was added to the [`InheritedPermissionsExtension`](api:SilverStripe\Security\InheritedPermissionsExtension) class so that now when duplicating any object that has the `InheritedPermissionsExtension` applied, the `GroupID` values in the `ViewerGroups` and `EditGroups` mapping tables will also be duplicated so that new object retains the same viewer and editor groups as the original.
+- And fields added to a model's `$summary_fields` configuration which are _not_ backed by database fields (such as method calls) will no longer be pulled through when `searchableFields()` calls back on it (i.e. because `$searchable_fields` configuration has not been explicitly declared). This means you do not need to explicitly declare `$searchable_fields` for models which should only use the summary fields to filter by.
 
 This release includes a number of bug fixes to improve a broad range of areas. Check the change logs for full details of these fixes split by module. Thank you to the community members that helped contribute these fixes as part of the release!