Rustdoc: overflowing supertraits should wrap, not scroll

[tgross35]

See e.g. https://docs.rs/num/latest/num/traits/int/trait.PrimInt.html, there are four horizontal pages listing the supertraits:

By contrast, rustfmt splits these into separate lines:

pub trait PrimInt:
    Sized
    + Copy
    + Num
    + NumCast
    + Bounded
    + PartialOrd
    + Ord
    + Eq
    + Not<Output = Self>
    + BitAnd<Output = Self>
    + BitOr<Output = Self>
    + BitXor<Output = Self>
    + Shl<usize, Output = Self>
    + Shr<usize, Output = Self>
    + CheckedAdd<Output = Self>
    + CheckedSub<Output = Self>
    + CheckedMul<Output = Self>
    + CheckedDiv<Output = Self>
    + Saturating
{
    /* ... */
}

Splitting into lines would make this easier to navigate. If vertical space consumption is a concern, maybe lengthy supertrait lists could be made collapsible (like methods currently are).

Somewhat related to #107283, but this seems to be a different variant of that problem.

[tgross35]

@rustbot label +T-rustdoc +T-rustdoc-frontend +A-rustdoc-ui -needs-triage