[clang-tidy][doc] Improve clang-tidy documentation

- Specify that the .clang-tidy file is in YAML format.
- Document the options that may be used in the .clang-tidy file,
- Add missing documentation for existing options (User).
- Fix spurious newline after the dash that comes after every
  command-line option. This was inconsistent with single-line
  descriptions, which lacked a newline. The description is now
  aligned with the dash and the corresponding command-line option,
  more visually pleasing.

This enables documenting upcoming global clang-tidy
configuration options.

Differential Revision: https://reviews.llvm.org/D141144