Long trait bounds don't get broken up in rustdoc

[clarfonthey]

Here's an example of some long bounds: num_traits::NumAssignOps

Copy-pasted here:

pub trait NumAssignOps<Rhs = Self>: AddAssign<Rhs> + SubAssign<Rhs> + MulAssign<Rhs> + DivAssign<Rhs> + RemAssign<Rhs> { }

Rustfmt, when given an adequately small width limit, will turn this into:

pub trait NumAssignOps<Rhs = Self>:
    AddAssign<Rhs>
    + SubAssign<Rhs>
    + MulAssign<Rhs>
    + DivAssign<Rhs>
    + RemAssign<Rhs>
{
}

It'd be nice if there were some way to make the rustdoc for these a bit less… awful, by default. Preferably without having to require that the user format their code accordingly.

[clarfonthey]

Going through old issues, updated this to include a more recent example + how rustfmt handles this, since rustfmt didn't handle it back when this issue was made.