Document how to deny warnings in examples

[Nemo157]

You can inject attributes into all doctests which can allow denying warnings, either outright (not recommended for the same reason as #![deny(warnings)] isn't)

#![doc(test(attr(deny(warnings))))]

or only in CI

#![cfg_attr(ci, doc(test(attr(deny(warnings))))]

with a runner running

> RUSTDOCFLAGS='--cfg ci' cargo test --doc

(with the same downsides of failing on new compilers like using RUSTFLAGS='-Dwarnings' cargo check).