Port over doc comments in route macros. (#2022)

Co-authored-by: Jonas Platte <jplatte@users.noreply.github.com>
Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-web-codegen/tests/trybuild/docstring-ok.rs

@@ -0,0 +1,17 @@
+use actix_web::{Responder, HttpResponse, App, test};
+use actix_web_codegen::*;
+
+/// Docstrings shouldn't break anything.
+#[get("/")]
+async fn index() -> impl Responder {
+    HttpResponse::Ok()
+}
+
+#[actix_web::main]
+async fn main() {
+    let srv = test::start(|| App::new().service(index));
+
+    let request = srv.get("/");
+    let response = request.send().await.unwrap();
+    assert!(response.status().is_success());
+}