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

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

utils/fetch-metadata/src/lib.rs

@@ -4,6 +4,8 @@
 
 //! Subxt utils fetch metadata.
 
+#![cfg_attr(docsrs, feature(doc_cfg))]
+
 // Internal helper macros
 #[macro_use]
 mod macros;