refactor: simplify docsrs cfg

[Frando]

Cleans up the feature-gate doc machinery left over from the quic-rpc origins.

• Drop the per-item doc(cfg(...)) annotations. Rustdoc now infers feature badges from the real #[cfg(...)] gates automatically. doc_auto_cfg was merged into doc_cfg in Rust 1.92 (rust-lang/rust#138907), so it is the default when feature(doc_cfg) is on. feature(doc_cfg) is nightly-only, but it is gated behind the irpc_docsrs cfg that only docs.rs sets. Regular builds never see it, so our MSRV stays at 1.91 on stable.
• Rename quicrpc_docsrs to irpc_docsrs, matching the crate name and the iroh_docsrs convention.

We keep a crate-specific cfg rather than the common docsrs because --cfg docsrs leaks into dependencies. A full doc build fails when cordyceps enables now-removed features behind #[cfg(docsrs)]. Using irpc_docsrs sidesteps that.