Explain why returning a box is an anti-pattern

Author: botahamec

clippy_lints/src/unused_box.rs

@@ -13,7 +13,9 @@ declare_clippy_lint! {
     ///
     /// ### Why is this bad?
     ///
-    /// It's better to just return `T` in these cases
+    /// It's better to just return `T` in these cases. The caller may not need
+    /// the value to be boxed, and it's expensive to free the memory once the
+    /// `Box<T>` been dropped.
     ///
     /// ### Example
     /// ```rust