Document which kind of changes are breaking

[m-ou-se]

We should document which kind of changes are breaking and cannot be made to stable APIs.

E.g. changing a function from fn(i32) to fn(impl Into<i32>) can be breaking, but relaxing a <T: Ord> to <T: PartialOrd> might be fine (maybe?).

[camelid]

And also, what level of breakage is acceptable. E.g., if a Crater run shows very few regressions, it's sometimes acceptable to make a breaking change.

[KodrAus]

We've got a section with some initial content here: https://github.com/rust-lang/std-dev-guide/tree/master/src/code-considerations/breaking-changes

There's still lots we could expand there!

[KodrAus]

I think it's usually inference that causes us the most pain in all kinds of changes that add trait impls and relax bounds.