Add docs for subxt-rpcs and fix example (#1954)

* Add docs for subxt-rpcs and fix example

* Add docs bits in Cargo.toml

* add missing docsrs cfg_attr

---------

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

rpcs/src/lib.rs

@@ -16,6 +16,8 @@
 //! The provided RPC client implementations can be used natively (with the default `native` feature
 //! flag) or in WASM based web apps (with the `web` feature flag).
 
+#![cfg_attr(docsrs, feature(doc_cfg))]
+
 #[cfg(any(
     all(feature = "web", feature = "native"),
     not(any(feature = "web", feature = "native"))