docs(build-rs): Make MSRV stick out

Author: epage

crates/build-rs/src/lib.rs

@@ -28,7 +28,15 @@ macro_rules! unstable {
 
 macro_rules! respected_msrv {
     ($ver:literal) => {
-        concat!("> MSRV: Respected as of ", $ver, ".")
+        concat!(
+            r#"<div class="warning">
+
+MSRV: Respected as of "#,
+            $ver,
+            r#".
+
+</div>"#
+        )
     };
 }