c2rust-refactor does not inform `clap` of transforms

[fw-immunant]

This means our --help output doesn't suggest any transforms to apply. See discussion in #1427.

[ahomescu]

We have them listed at https://c2rust.com/manual/c2rust-refactor/commands.html, but I'm not sure how current that list is. Plus it makes sense for the tool itself to show you some useful information, instead of having to visit a website.

[fw-immunant]

Yeah, #1427 gets the tool printing the list when an invalid transform is given; this bug is just tracking doing it the proper clean way through the clap library itself, which is lower priority.