Improve docs in taproot module

As has been done in other places in the codebase; improve the docs in
the `taproot` module by doing:

- Use full sentences (capital letters + full stops)
- Use back ticks and links for types where appropriate
- Fix grammar
- Fix stale docs
- Use third person for describing functions
- Use 100 character line width
- Use markdown sections (`# Examples`, `# Returns`) where appropriate
- Separate brief heading from extended description when appropriate
- Use `///` for all functions/types (both private and public)

I also did:

- Build the docs and check all the links
- Read all the built docs, check for sanity and pretty-ness

Author: tcharding