Document map_clone known problems #498

(cherry picked from commit ada0d2c548)

clippy_lints/src/map_clone.rs

@@ -27,7 +27,9 @@ pub struct Pass;
 ///
 /// **Why is this bad?** Readability, this can be written more concisely
 ///
-/// **Known problems:** None.
+/// **Known problems:** Sometimes `.cloned()` requires stricter trait
+/// bound than `.map(|e| e.clone())` (which works because of the coercion).
+/// See [#498](https://github.com/rust-lang-nursery/rust-clippy/issues/498).
 ///
 /// **Example:**
 ///