Add example of false positive to PTR_ARG docs.

Fixes rust-lang#214

clippy_lints/src/ptr.rs

@@ -36,14 +36,27 @@ declare_clippy_lint! {
     /// argument may also fail to compile if you change the argument. Applying
     /// this lint on them will fix the problem, but they may be in other crates.
     ///
+    /// One notable example of a function that may cause issues, and which cannot
+    /// easily be changed due to beinng in the standard library is `Vec::contains`.
+    /// when called on a `Vec<Vec<T>>`. If a `&Vec` is passed to that method then
+    /// it will compile, but if a `&[T]` is passed then it will not compile.
+    ///
+    /// ```ignore
+    /// fn cannot_take_a_slice(v: &Vec<u8>) -> bool {
+    ///     let vec_of_vecs: Vec<Vec<u8>> = some_other_fn();
+    ///
+    ///     vec_of_vecs.contains(v)
+    /// }
+    /// ```
+    ///
     /// Also there may be `fn(&Vec)`-typed references pointing to your function.
     /// If you have them, you will get a compiler error after applying this lint's
     /// suggestions. You then have the choice to undo your changes or change the
     /// type of the reference.
     ///
     /// Note that if the function is part of your public interface, there may be
-    /// other crates referencing it you may not be aware. Carefully deprecate the
-    /// function before applying the lint suggestions in this case.
+    /// other crates referencing it, of which you may not be aware. Carefully 
+    /// deprecate the function before applying the lint suggestions in this case.
     ///
     /// **Example:**
     /// ```ignore