[contracts] Add docs generator for the contracts API to the #[define_env] macro (#13032)

* macro to expand traits for host functions documentation

* other way: same Doc trait in seal modules

* added docs for macro, and remove `doc` attribute

* fmt

* Apply suggestions from code review

Co-authored-by: Alexander Theißen <alex.theissen@me.com>

* make docs to be generated into re-exported `api_doc` module; fix
unrelated elder docs;

* make it compile without `doc` attr passed to macro

* make alias functions indicated explicitly in docs

* tidy up docs

* refactored a bit

* macro to auto-add doc warning for unstable functions

* invoke macro with no doc generation by default

* addressed review comments

* hide api_doc module behind cfg(doc)

Co-authored-by: Alexander Theißen <alex.theissen@me.com>

substrate/frame/contracts/src/wasm/mod.rs

@@ -28,6 +28,10 @@ pub use crate::wasm::{
 	prepare::TryInstantiate,
 	runtime::{CallFlags, Environment, ReturnCode, Runtime, RuntimeCosts},
 };
+
+#[cfg(doc)]
+pub use crate::wasm::runtime::api_doc;
+
 use crate::{
 	exec::{ExecResult, Executable, ExportedFunction, Ext},
 	gas::GasMeter,
@@ -144,7 +148,7 @@ impl<T: Config> PrefabWasmModule<T> {
 	/// Create the module by checking and instrumenting `original_code`.
 	///
 	/// This does **not** store the module. For this one need to either call [`Self::store`]
-	/// or [`<Self as Executable>::execute`].
+	/// or [`<Self as Executable>::execute`][`Executable::execute`].
 	pub fn from_code(
 		original_code: Vec<u8>,
 		schedule: &Schedule<T>,
@@ -164,7 +168,8 @@ impl<T: Config> PrefabWasmModule<T> {
 
 	/// Store the code without instantiating it.
 	///
-	/// Otherwise the code is stored when [`<Self as Executable>::execute`] is called.
+	/// Otherwise the code is stored when [`<Self as Executable>::execute`][`Executable::execute`]
+	/// is called.
 	pub fn store(self) -> DispatchResult {
 		code_cache::store(self, false)
 	}