Add XTS as supporting graceful shutdown, per ochaloup@redhat.com's feedback:

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I checked the document and I can't other specific issue that should be included. Despite that - maybe it would be good to mention that graceful shutdown[1] is working the XTS component too.

XTS is strongly coupled with the Transaction component (and especially in this case) thus it could be just enough to add to the sentence under Transactions release notes that the graceful shutdown works for XTS too. Or if it's not the right way I would think that XTS could be mentioned in the document as supporting graceful shutdown.
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