Add docs for `mir::BasicBlock`

[camelid]

@rustbot modify labels: A-mir T-doc C-enhancement E-easy

[camelid]

I would be interested in doing this :)

@rustbot claim

[jyn514]

Should this issue be on rustc-dev-guide?

[camelid]

I'm not sure if it's covered there, but I meant in terms of a doc comment on the struct.

[jyn514]

Ok, take a look around https://rustc-dev-guide.rust-lang.org/mir/index.html?highlight=basicblock#mir-data-types then. But you might want to discuss around Zulip what the convention is for doc-comments.

[camelid]

Hmm, I think most of the items in MIR have doc comments. I can ask on Zulip though.