chore: add example for balance AccountStore

frame/balances/src/lib.rs

@@ -488,8 +488,29 @@ pub mod pallet {
 	#[pallet::getter(fn total_issuance)]
 	pub type TotalIssuance<T: Config<I>, I: 'static = ()> = StorageValue<_, T::Balance, ValueQuery>;
 
-	/// The balance of an account.
+	/// The Balances pallet example of storing the balance of an account.
 	///
+	/// # Example
+	///
+	/// ```nocompile
+	///  impl pallet_balances::Config for Runtime {
+	///    type AccountStore = StorageMapShim<Self::Account<Runtime>, frame_system::Provider<Runtime>, AccountId, Self::AccountData<Balance>>
+	///  }
+	/// ```
+	///
+	/// You can also store the balance of an account in the `System` pallet.
+	///
+	/// # Example
+	///
+	/// ```nocompile
+	///  impl pallet_balances::Config for Runtime {
+	///   type AccountStore = System
+	///  }
+	/// ```
+	///
+	/// But this comes with tradeoffs, storing account balances in the system pallet stores
+	/// `frame_system` data alongside the account data contrary to storing account balances in the
+	/// `Balances` pallet, which uses a `StorageMap` to store balances data only.
 	/// NOTE: This is only used in the case that this pallet is used to store balances.
 	#[pallet::storage]
 	pub type Account<T: Config<I>, I: 'static = ()> = StorageMap<