doc: Add documentation about expert mode in XTS widget

Part of story #39019 Switch mode in Cross tracker search widget

languages/en/user-guide/tql.rst

@@ -259,3 +259,44 @@ Example
     //Returns all artifacts with 'documentation' in the title that have been submitted
     //by user 'alice' and are assigned to the viewing user (for example 'bob')
     //or user 'charles'.
+
+Expert mode
+-----------
+
+.. attention::
+
+  This part is still work in progress, future releases may break your report.
+  You can follow advancement in the associated epic https://tuleap.net/plugins/tracker/?aid=37567
+
+Cross-tracker search widget is also available in expert mode allowing you to use extended syntax of TQL.
+
+In this extended syntax of TQL you can choose which fields you want to display on the widget through ``SELECT`` syntax:
+
+::
+
+    SELECT @pretty_title, @status, open_date WHERE @assigned_to = MYSELF()
+    // Returns all artifacts assigned to me and display its title, status and opening date.
+
+When using Cross-tracker search expert mode, you must use ``SELECT`` syntax with at least one field and a condition after the ``WHERE``.
+
+TQL ``SELECT`` syntax allow you to select on the same fields allowed for the condition plus some special fields:
+
+Semantics and always there fields:
+ * ``@id`` Artifact id
+ * ``@submitted_on`` Artifact creation datetime
+ * ``@last_update_date`` Artifact last update datetime
+ * ``@submitted_by`` User who created the artifact
+ * ``@last_update_by`` User who performed last update of the artifact
+ * ``@title`` Artifact title semantic
+ * ``@description`` Artifact description semantic
+ * ``@status`` Artifact status semantic
+ * ``@assigned_to`` Artifact assignee/contributor semantic
+
+Fields short name
+
+Special fields:
+ * ``@project.name``
+ * ``@tracker.name``
+ * ``@pretty_title`` It's equivalent to the Artifact column of classic Cross-tracker search widget (or the title of the artifact view).
+
+To switch a widget to this mode, you should use the REST API ``PUT /cross_tracker_reports/{id}``. The body of your request should contains the key ``report_mode`` with the value ``expert``.