Auto merge of #88522 - camelid:box-paren-output, r=jyn514

Auto merge of #88522 - camelid:box-paren-output, r=jyn514

rustdoc: Box `GenericArgs::Parenthesized.output`

Split out from #88379.

This reduces the size of `GenericArgs` from 104 bytes to 56 bytes,
essentially reducing it by half.

`GenericArgs` is one of the fields of `PathSegment`, so this should
reduce the amount of memory allocated for `PathSegment`s in the cases
where the generics are not for a `Fn`, `FnMut`, or `FnOnce` trait.

r? `@jyn514`