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/tx/mod.rs

@@ -9,6 +9,8 @@
 //! additional and signed extra parameters are used when constructing an extrinsic, and is a part
 //! of the chain configuration (see [`crate::config::Config`]).
 
+use crate::macros::cfg_substrate_compat;
+
 mod signer;
 mod tx_client;
 mod tx_payload;
@@ -16,8 +18,9 @@ mod tx_progress;
 
 // The PairSigner impl currently relies on Substrate bits and pieces, so make it an optional
 // feature if we want to avoid needing sp_core and sp_runtime.
-#[cfg(feature = "substrate-compat")]
-pub use self::signer::PairSigner;
+cfg_substrate_compat! {
+    pub use self::signer::PairSigner;
+}
 
 pub use self::{
     signer::Signer,