update genesis_config related docs and tests and error messages (#1642)

Follow-up to https://github.com/paritytech/substrate/pull/14306.

I hope this also showcases the important message of: **It is really not
that hard to make the examples codes in rust-docs compile, and therefore
remain correct. Please embrace this :)**

It moves the documentation of proc macros to their re-export, such that
can link other items in frame-support. This is a patter that we should
embrace for all of macro docs, and apply in PRs like
https://github.com/paritytech/substrate/pull/13987 as well.

---------

Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: command-bot <>

substrate/frame/support/src/lib.rs

@@ -2188,10 +2188,80 @@ pub mod pallet_macros {
 	pub use frame_support_procedural::{
 		call_index, compact, composite_enum, config, constant,
 		disable_frame_system_supertrait_check, error, event, extra_constants, generate_deposit,
-		generate_store, genesis_build, genesis_config, getter, hooks, import_section, inherent,
-		no_default, no_default_bounds, origin, pallet_section, storage, storage_prefix,
-		storage_version, type_value, unbounded, validate_unsigned, weight, whitelist_storage,
+		generate_store, getter, hooks, import_section, inherent, no_default, no_default_bounds,
+		origin, pallet_section, storage, storage_prefix, storage_version, type_value, unbounded,
+		validate_unsigned, weight, whitelist_storage,
 	};
+
+	/// Allows you to define the genesis configuration for the pallet.
+	///
+	/// Item is defined as either an enum or a struct. It needs to be public and implement the
+	/// trait [`frame_support::traits::BuildGenesisConfig`].
+	///
+	/// See [`genesis_build`] for an example.
+	pub use frame_support_procedural::genesis_config;
+
+	/// Allows you to define how the state of your pallet at genesis is built. This
+	/// takes as input the `GenesisConfig` type (as `self`) and constructs the pallet's initial
+	/// state.
+	///
+	/// The fields of the `GenesisConfig` can in turn be populated by the chain-spec.
+	///
+	/// ## Example:
+	///
+	/// ```
+	/// #[frame_support::pallet]
+	/// pub mod pallet {
+	/// # 	#[pallet::config]
+	/// # 	pub trait Config: frame_system::Config {}
+	/// # 	#[pallet::pallet]
+	/// # 	pub struct Pallet<T>(_);
+	/// # 	use frame_support::traits::BuildGenesisConfig;
+	///     #[pallet::genesis_config]
+	///     #[derive(frame_support::DefaultNoBound)]
+	///     pub struct GenesisConfig<T: Config> {
+	///         foo: Vec<T::AccountId>
+	///     }
+	///
+	///     #[pallet::genesis_build]
+	///     impl<T: Config> BuildGenesisConfig for GenesisConfig<T> {
+	///         fn build(&self) {
+	///             // use &self to access fields.
+	///             let foo = &self.foo;
+	///             todo!()
+	///         }
+	///     }
+	/// }
+	/// ```
+	///
+	/// ## Former Usage
+	///
+	/// Prior to <https://github.com/paritytech/substrate/pull/14306>, the following syntax was used.
+	/// This is deprecated and will soon be removed.
+	///
+	/// ```
+	/// #[frame_support::pallet]
+	/// pub mod pallet {
+	/// #     #[pallet::config]
+	/// #     pub trait Config: frame_system::Config {}
+	/// #     #[pallet::pallet]
+	/// #     pub struct Pallet<T>(_);
+	/// #     use frame_support::traits::GenesisBuild;
+	///     #[pallet::genesis_config]
+	///     #[derive(frame_support::DefaultNoBound)]
+	///     pub struct GenesisConfig<T: Config> {
+	/// 		foo: Vec<T::AccountId>
+	/// 	}
+	///
+	///     #[pallet::genesis_build]
+	///     impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
+	///         fn build(&self) {
+	///             todo!()
+	///         }
+	///     }
+	/// }
+	/// ```
+	pub use frame_support_procedural::genesis_build;
 }
 
 #[deprecated(note = "Will be removed after July 2023; Use `sp_runtime::traits` directly instead.")]