Merge bitcoindevkit/bdk#633: Additional comments for `TransactionDetails`.

77c7d0aae944dbd5a44ca9b8f239db0c9edcd4f8 Additional comments for `TransactionDetails`. (志宇)

Pull request description:

  ### Description

  I'm not sure if this is needed or helpful, but this PR adds comments to describe how the `sent` and `received` fields of `TransactionDetails` are calculated.

  I wasn't sure how it was done until I looked deeper into the codebase (but maybe I am too much of a beginner and this is common sense for most).

  ### 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:
  danielabrozzoni:
    re-ACK 77c7d0aae944dbd5a44ca9b8f239db0c9edcd4f8

Tree-SHA512: 8d29d249a70bc2d0631078b6772c5543bdc61ee43df3810ab666f5e97ca59b0d4cfc8acad14bbaf8674baba319f24fa2781a42740ca42bccd9688831aaedea72