task: document cancel safety of LocalSet::run_until (#6147)

Closes: #6122

tokio/src/task/local.rs

@@ -602,6 +602,10 @@ impl LocalSet {
     /// will remain on the local set, and will be driven on subsequent calls to
     /// `run_until` or when [awaiting the local set] itself.
     ///
+    /// # Cancel safety
+    ///
+    /// This method is cancel safe when `future` is cancel safe.
+    ///
     /// # Examples
     ///
     /// ```rust