Document the `run_make_support` library

The support library has some sparse doc comments via ./x doc src/tools/run-make-support, but it really needs some better high-level docs to help test writers (otherwise the support library is very hard to discover). There also needs to be crosslinks from rustc-dev-guide back to these docs.

https://rust-lang.zulipchat.com/#narrow/stream/421156-gsoc/topic/Project.3A.20Rewriting.20Makefile.20Tests.20Using.20Rust/near/443513926 reproduced here for convenience:

I'm thinking that I should add some docs for the support library, for stuff like:

• Assumptions (e.g. paths relative to the working directory of the test being run)
• Key API design: the Command wrapper and CompletedProcess etc.
• Synposis of common Command and CompletedProcess APIs
• Guidance for adding new command wrappers, adding new dependencies, when to uplift things from tests to the support library etc.
• Recommendations: error reporting
• Reminder to augment and update the changelog