Add links to std::char::REPLACEMENT_CHARACTER from docs.

There are a few places where we mention the replacement character in the
docs, and it could be helpful for users to utilize the constant which is
available in the standard library, so let’s link to it!

src/libstd/path.rs

@@ -1737,9 +1737,11 @@ impl Path {
 
     /// Converts a `Path` to a [`Cow<str>`].
     ///
-    /// Any non-Unicode sequences are replaced with U+FFFD REPLACEMENT CHARACTER.
+    /// Any non-Unicode sequences are replaced with
+    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD].
     ///
     /// [`Cow<str>`]: ../borrow/enum.Cow.html
+    /// [U+FFFD]: ../char/constant.REPLACEMENT_CHARACTER.html
     ///
     /// # Examples
     ///