1.15.1 docs.rs build failed due to doc_auto_cfg stabilization

[djc]

See https://docs.rs/crate/aws-lc-rs/latest/builds/2676635.

Example mitigation in rustls:

• Privatise docsrs cfg flag; fix nightly rustls/rustls#2682

[justsmth]

Thanks for the notification and links. I'll also take note to add checking docs.rs to our release checklist.