This PR is a continuation of the work in https://github.com/bitcoin/bitcoin/pull/12892.

Main motivations:
- There is currently no test coverage for the getaddressinfo `labels` response. Coverage here is a prerequisite before deprecating the `label` response or adding multiple labels per address.
- `bitcoin-cli help getaddressinfo` returns a few content errors, difficult-to-read formatting, and no explanation why it returns both `label` and `labels` and how they relate, which can be confusing for application developers.

Changes by order of commits:
- [x] improve/fix getaddressinfo RPCHelpman layout formatting
- [x] improve/fix getaddressinfo RPCHelpman content
- [x] clarify the `label` and `labels` fields in getaddressinfo RPCHelpman
- [x] update getaddressinfo RPCExamples addresses to bech32
- [x] add getaddressinfo code docs
- [x] add a `listlabels` test assertion in wallet_labels.py
- [x] add missing getaddressinfo `labels` test coverage and improve the existing `label` tests

Here are gists of the CLI help output:
[`bitcoin-cli help getaddressinfo` before this PR](https://gist.github.com/jonatack/022af5221a85c069780359a22643c810)
[`bitcoin-cli help getaddressinfo` after this PR](https://gist.github.com/jonatack/4ee5f6abc62a3d99269570206a5f90ba)

It seems we ought to begin a deprecation process for the getaddressinfo `label` field? If yes, I have a follow-up ready. _--> EDIT: Deprecation follow-ups #17578 and #17585 now build on this PR._

ninja all check check-functional