Merge bitcoindevkit/bdk#1596: doc(wallet): Add docs to explain the lookahead

Merge bitcoindevkit/bdk#1596: doc(wallet): Add docs to explain the lookahead

028f687b21872b66a2907701e5a8e143d0f867a0 doc(wallet): Add docs to explain the lookahead (valued mammal)

Pull request description:

  Adds clarifying language to `CreateParams` and `LoadParams` regarding the `lookahead` parameter. Commit 028f687b21872b66a2907701e5a8e143d0f867a0 also includes some minor documentation fixes.

  If anyone is aware of any more documentation flaws that need attention I'm happy to add them here.

  cc #1540

  ### 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 fmt` and `cargo clippy` before committing

ACKs for top commit:
  LagginTimes:
    ACK 028f687b21872b66a2907701e5a8e143d0f867a0
  oleonardolima:
    ACK 028f687b21872b66a2907701e5a8e143d0f867a0
  evanlinjin:
    ACK 028f687b21872b66a2907701e5a8e143d0f867a0

Tree-SHA512: 971d09652948ed2e2dc86d255cfd18607d96b0806aa0e990190cd1d7035c6660ea2ac1092ef2c6c209e61920c0d4ff9d8c0a900bcc74a8662546d284fec3218f