Add docs

Author: AndreiEres

polkadot/node/core/pvf/src/host.rs

@@ -140,6 +140,10 @@ impl ValidationHost {
 			.map_err(|_| "the inner loop hung up".to_string())
 	}
 
+	/// Sends validation code hashes with executor params to the validation host to check if
+	/// appropriate artifacts were prepared. Hashes for unprepared artifacts are sent back.
+	///
+	/// Returns an error if the request cannot be sent to the validation host, i.e. if it shut down.
 	pub async fn ensure_pvf(
 		&mut self,
 		code_hashes: Vec<ValidationCodeHash>,