add to known problems

Author: DevinR528

clippy_lints/src/macro_use.rs

@@ -12,7 +12,7 @@ declare_clippy_lint! {
     /// **Why is this bad?** Since the Rust 2018 edition you can import
     /// macro's directly, this is considered idiomatic.
     ///
-    /// **Known problems:** None.
+    /// **Known problems:** This lint does not generate an auto-applicable suggestion.
     ///
     /// **Example:**
     /// ```rust