ensure lint-docs build for allow-by-default must_not_suspend

Author: guswynn

compiler/rustc_lint_defs/src/builtin.rs

@@ -322,6 +322,7 @@ declare_lint! {
     ///
     /// ```rust
     /// #![feature(must_not_suspend)]
+    /// #![warn(must_not_suspend)]
     ///
     /// #[must_not_suspend]
     /// struct SyncThing {}