Merge bitcoindevkit/bdk#2376: docs(chain): document that `apply_block` trusts the...

2cce69780e6ca215c450cb9d8ebd1c752339c99b docs(chain): document that `apply_block` trusts the given block (志宇)

Pull request description:

  Fixes #2278. Supersedes #2373.

  Documents that `apply_block` and `apply_block_relevant` don't validate the block (chain membership or merkle root), so the caller must supply it from a trusted source. No `debug_assert!`: a bad block comes from a bad source at runtime, where the assert is compiled out.

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