docs(chain): state that an anchor names the confirming block

docs(chain): state that an anchor names the confirming block

The `Anchor` trait doc claimed an anchored transaction "could also mean
transaction A is confirmed in a parent block of B". No chain source in
this repo produces such an anchor: electrum only builds one after
`validate_merkle_proof` succeeds, esplora uses `TxStatus`'s confirming
block, and `TxPosInBlock` carries the transaction's index within that
block. The framing only leaves ambiguity for a case that does not occur.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NND8BhEmYTYdLRv4dbUUro