local feature is not additive

[epage]

Describe the bug
If one package is using tame-index and another package activates feature = "local", then the build will break

Expected behavior
Build should not break; features should be additive

Additional context
Potential solutions

• Make ComboIndexCache non_exhaustive
• Have the Local variant but ensure LocalRegistry can never be constructed

[epage]

It'd also be useful to use #![cfg_attr(docsrs, feature(doc_auto_cfg))] like I do with clap. While reviewing crate-ci/cargo-release#693, I was scratching my head for a bit as to why I see LocalRegistry in the docs but it wasn't being matched in the PR.