Document when and how to specialize for optimization

[m-ou-se]

A lot of iterator adapters have specialized implementations for when the underlying iterator implements e.g. TrustedRandomAccess.Traits like that are marked with #[rustc_specialization_marker] or #[rustc_unsafe_specialization_marker]. We should document when to use this, and what it means for stability guarantees.

cc @matthewjasper

[KodrAus]

We've got a section here on specialization we can expand with #[rustc_specialization_marker] and whatnot: https://github.com/rust-lang/std-dev-guide/blob/master/src/code-considerations/using-unstable-lang/specialization.md