Document features for docs.rs

[heaths]

Resolves #2223

[heaths]

To note, I added the unexpected_cfgs lint despite docsrs not seemingly needing it, but according to https://doc.rust-lang.org/nightly/rustc/check-cfg.html#well-known-names-and-values at the time of this comment, itself referencing 2025-01-02T as the time the list was known, docsrs was not one of them though it didn't warn of an unexpected_cfg lint despite tried, made-up values emitting that lint e.g., docsr (no trailing "s") showed that lint. I wanted to make sure it was explicitly allowed to avoid trouble later.