Docs.rs feature `doc_auto_cfg` has been removed a while ago

[eirnym]

This renders documentation to fail to build: https://docs.rs/crate/reqsign/latest/builds/4062142

AFAIK this feature is merged with doc_cfg and be sure that recent nightly is used as there were some implementation gaps in the past.

Also cargo docs-rs does a good approximation of what docs.rs does, so it would be easier to catch such issues.