Document the Call derive macro (#137)

* Document the `Call` derive macro

* Obey the fmt

* Update proc-macro/src/lib.rs

Co-authored-by: Andrew Jones <ascjones@gmail.com>

* Review feedback

Co-authored-by: Andrew Jones <ascjones@gmail.com>

examples/submit_and_watch.rs

@@ -16,7 +16,10 @@
 
 use sp_keyring::AccountKeyring;
 use substrate_subxt::{
-    balances::*,
+    balances::{
+        TransferCallExt,
+        TransferEventExt,
+    },
     ClientBuilder,
     DefaultNodeRuntime,
     PairSigner,