Merge rust-bitcoin/rust-bitcoin#912: Improve docs in taproot module

c25eddd Remove unnecessary documentation (Tobin Harding)
8631474 Improve docs in taproot module (Tobin Harding)

Pull request description:

  I should have done this PR a month ago, my bad. This one is kind of important IMO because we are going to have so many people looking at this part of the code soon as we release.

  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

  Its all in one patch, I couldn't really tease it apart. I can try a bit harder if it proves too annoying to review.

ACKs for top commit:
  sanket1729:
    ACK c25eddd
  dr-orlovsky:
    ACK c25eddd
  apoelstra:
    ACK c25eddd

Tree-SHA512: 72f35bf8779392060388db985df5abc42a89796eaad1eafd08ea50b635d469fbd07a53ff253cdf27ad4d4baed7d37cec6ea1da1aece3672b9447f87181e218f8

Author: dr-orlovsky