Fix large_types_passed_by_value example and improve docs

Author: cauebs

clippy_lints/src/pass_by_ref_or_value.rs

@@ -61,8 +61,9 @@ declare_clippy_lint! {
     /// because that might induce API breakage, if the parameter is declared as mutable,
     /// or if the argument is a `self`.
     ///
-    /// **Why is this bad?** Arguments passed by value might require a shallow copy,
-    /// which can be expensive and can be easily improved with passing by reference.
+    /// **Why is this bad?** Arguments passed by value might result in an unnecessary
+    /// shallow copy, taking up more space in the stack and requiring a call to
+    /// `memcpy`, which which can be expensive.
     ///
     /// **Example:**
     ///
@@ -73,9 +74,11 @@ declare_clippy_lint! {
     /// // Bad
     /// fn foo(v: TooLarge) {}
     /// ```
-    ///
     /// ```rust
-    /// // Better
+    /// #[derive(Clone, Copy)]
+    /// struct TooLarge([u8; 2048]);
+    ///
+    /// // Good
     /// fn foo(v: &TooLarge) {}
     /// ```
     pub LARGE_TYPES_PASSED_BY_VALUE,