build_helper isn't documented in the rustc internal docs

[lolbinarycat]

Location

https://doc.rust-lang.org/nightly/nightly-rustc

Summary

the build_helper crate is not documented. this makes working on things like bootstrap more difficult. it is not used by the compiler directly, but neither is run_make_support, which is documented.

[jieyouxu]

This needs a doc step in bootstrap, previously Chris (I think?) added doc step for run_make_support which is why that exists.