Merge bitcoindevkit/bdk#686: doc: Document that list_transactions() might return...

Merge bitcoindevkit/bdk#686: doc: Document that list_transactions() might return unsorted txs

7fdacdbad40f4e9f6726b064d8eb4d93789e9990 doc: Document that list_transactions() might return unsorted txs, show how to sort them if needed (w0xlt)

Pull request description:

  This PR documents that `list_transactions()` might return unsorted transaction and shows how to sort them if needed.

  Closes #518.

ACKs for top commit:
  danielabrozzoni:
    re-ACK 7fdacdbad40f4e9f6726b064d8eb4d93789e9990

Tree-SHA512: 83bec98e1903d6dc6b8933e8994cb9d04aad059cee8a7b8e1e3a322cf52511364b36d0cd6be1c8cb1fd82c67f8be5a262bbd2c76e30b24eb4097c30f38aa8b10