Merge bitcoindevkit/bdk#1968: bench: Add `reindex_tx_graph` benchmark

f51f5b5659941a68b74e5b142296a8aaf9665afa docs(chain): Improve API docs (valued mammal)
fcf38349d895b0c627b4149e448a29468ed629b4 test(keychain_txout): test spk cache (valued mammal)
e07ec1cefbb6cc973398d4281bfde58c8c15cc4b bench(chain): Add `reindex_tx_graph` benchmark (valued mammal)

Pull request description:

  This PR contains the following changes:

  - Add benchmark `reindex_tx_graph`. Run with `cargo bench -p bdk_chain --bench indexer`.
  - Add unit test to `keychain_txout` module to test behavior of spk-cache.
  - Fixup a few doc comments.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo +nightly fmt` and `cargo clippy` before committing

ACKs for top commit:
  ValuedMammal:
    ACK f51f5b5659941a68b74e5b142296a8aaf9665afa

Tree-SHA512: 4d6826a825fa40fa13bd622ed6cff49366c541a7e0a7035a45fbeb98853d693ddef7fab113f6d77259fd75eddfc91623834363e028235fb26336c8865fccb02b