I have access to the `clap-man` name but we use `clap_*`. Rather than
people getting mixed up on which is supposed to use, we went with a
different name.
This is an initial implementation with plenty of room to grow, including
- Allowing pulling out a subset of the generated man page for greater customization
- Subcommand handling
- Extra sections
- Consolidate argument formatter after #2914Fixes#552