generate docs for feature gated items (#1332)

* generate docs for feature-gated items on crates.io

* add newline in Cargo.toml

* unify newlines

* fix clippy

* introduce macros for features

* commit missing file

* Update subxt/src/lib.rs

* make it compile

subxt/src/error/mod.rs

@@ -8,8 +8,9 @@ mod dispatch_error;
 
 use core::fmt::Debug;
 
-#[cfg(feature = "unstable-light-client")]
-pub use crate::client::LightClientError;
+crate::macros::cfg_unstable_light_client! {
+    pub use crate::client::LightClientError;
+}
 
 // Re-export dispatch error types:
 pub use dispatch_error::{
@@ -73,6 +74,7 @@ pub enum Error {
     Unknown(Vec<u8>),
     /// Light client error.
     #[cfg(feature = "unstable-light-client")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "unstable-light-client")))]
     #[error("An error occurred but it could not be decoded: {0}")]
     LightClient(#[from] LightClientError),
     /// Other error.