Add doc for auto-generated code (#7201)

* Add doc for auto-generated code

* Update utils/wasm-builder-runner/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/utils/wasm-builder-runner/src/lib.rs

@@ -403,6 +403,8 @@ fn create_project(
 		project_folder.join("src/main.rs"),
 		format!(
 			r#"
+				//! This is automatically generated code by `substrate-wasm-builder`.
+
 				use substrate_wasm_builder::build_project_with_default_rustflags;
 
 				fn main() {{