Merge bitcoindevkit/bdk#2350: docs(esplora): explain prevout trust assumptions

Merge bitcoindevkit/bdk#2350: docs(esplora): explain prevout trust assumptions

e634a5ab935a3afe9a0a683f79d54989508a16ef docs(esplora): explain prevout trust assumptions (hassan)

Pull request description:

  ### Description

  Refs #2343

  Esplora `sync` and `full_scan` insert server-reported previous outputs into `TxUpdate::txouts` without fetching the parent transactions to verify them. If a parent isn't in the `TxGraph`, fee and sent/received figures rely on these unverified values.

  This adds a "Trust assumptions" section to the `bdk_esplora` README saying so, and links `sync`/`full_scan` (blocking and async) to it. Docs only; no behaviour change.

  ### Changelog notice

  #### Added

  - `bdk_esplora`: document that the previous outputs `sync` and `full_scan` add to `TxUpdate::txouts` are server-reported and unverified (new "Trust assumptions" section in the crate docs). #2350

  ### Checklists

  #### All Submissions:

  * [x] I followed the contribution guidelines

ACKs for top commit:
  evanlinjin:
    ACK e634a5ab935a3afe9a0a683f79d54989508a16ef

Tree-SHA512: d96822f29a0ef012fa1acc272ca91c1fa6979c0182809be45edd45b5c85213e11f3732275411b5d62c6bb9f16f252b1ad535df2a93247262116ac55a39a01c80