swift234

[docs] More documentation for lit substitutions

Add explanations for lit substitutions marked as "FIXME" in the testing
documentation.

- Place all `%target-*` substitutions in the same section of the
  documentation.
- Remove substitutions that are no longer available or in use, such as
  `%llvm-opt` and `%leaks-runner`.
- Use uniform spacing between all substitutions bullet points.