doc: Add documentation on XTS (#1368)

* doc: Add documentation on XTS

* fix: Remove separate section for duck typing XTS

languages/en/user-guide/tql.rst

@@ -186,6 +186,20 @@ Lists bound to users:
  * ``@last_update_by``: the "Last Updated by" dynamic field. It behaves like a list and can have only a single value at a time.
  * ``@assigned_to``: the "Contributor/assignee"`` semantic. It behaves like a list and can have multiple values at a time (multiple users assigned to an artifact).
 
+You can also search on any custom field with its name as long as there is at least one Tracker with a compatible definition. We consider that 2 fields from 2 Trackers are compatible if:
+ * You can see both fields
+ * They have the same name
+ * Their types are compatible
+
+Compatible field types:
+ * Numerics: integer and float
+ * Lists with same bind (user, user group, static)
+ * String and text
+ * Date
+ * Date time
+
+TQL behavior to search on these fields is the same as for classic single tracker search.
+
 Preconditions for multi-tracker search
 --------------------------------------