Document tidy and bootstrap

[albertlarsan68]

Location

Most of the tidy tool (located at src/tools/tidy), and most of the bootstrap source (located at src/bootstrap)

Summary

Actually, adding a new tidy check means digging through existing code to find how it works.
I propose adding doc-comments to the tidy items, in order to get the new contributors to understand it better.
We should also do it for the bootstrap crate, since it is a base building block of Rust, but currently is very opaque in how it works.

Future possibilities

We can also post the rendered docs to doc.rust-lang.org, but this is a later step that is being made possible by documenting the current items.

[albertlarsan68]

@rustbot label +A-bootstrap

[Teapot4195]

@rustbot claim

i've noticed most of tidy already has crate level docs, so my guess is that we should document the functions in the crates (found in tidy and bootstrap)?

[jyn514]

We can also post the rendered docs to doc.rust-lang.org, but this is a later step that is being made possible by documenting the current items.

I think we can start doing that even before writing manual documentation, the autogenerated docs from rustdoc are already quite helpful.

[Teapot4195]

Sounds good, I'll open a pr for that.