[9.1] Update templates.md with fleet-synced-integrations

manage-data/data-store/templates.md

@@ -42,6 +42,7 @@ If you use {{fleet}} or {{agent}}, assign your index templates a priority lower
 * Use a non-overlapping index pattern.
 * Assign templates with an overlapping pattern a `priority` higher than `500`. For example, if you don’t use {{fleet}} or {{agent}} and want to create a template for the `logs-*` index pattern, assign your template a priority of `500`. This ensures your template is applied instead of the built-in template for `logs-*-*`.
 * To avoid naming collisions with built-in and Fleet-managed index templates, avoid using `@` as part of the name of your own index templates.
+* Fleet uses indices `fleet-synced-integrations*` for a feature, avoid using this name to avoid naming collisions.
 
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