Add missing urls for Sum and Product traits

src/libcore/iter/traits.rs

@@ -556,10 +556,14 @@ impl<'a, I: ExactSizeIterator + ?Sized> ExactSizeIterator for &'a mut I {}
 
 /// Trait to represent types that can be created by summing up an iterator.
 ///
-/// This trait is used to implement the `sum` method on iterators. Types which
-/// implement the trait can be generated by the `sum` method. Like
-/// `FromIterator` this trait should rarely be called directly and instead
-/// interacted with through `Iterator::sum`.
+/// This trait is used to implement the [`sum()`] method on iterators. Types which
+/// implement the trait can be generated by the [`sum()`] method. Like
+/// [`FromIterator`] this trait should rarely be called directly and instead
+/// interacted with through [`Iterator::sum()`].
+///
+/// [`sum()`]: ../../std/iter/trait.Sum.html#tymethod.sum
+/// [`FromIterator`]: ../../std/iter/trait.FromIterator.html
+/// [`Iterator::sum()`]: ../../std/iter/trait.Iterator.html#method.sum
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
 pub trait Sum<A = Self>: Sized {
     /// Method which takes an iterator and generates `Self` from the elements by
@@ -571,10 +575,14 @@ pub trait Sum<A = Self>: Sized {
 /// Trait to represent types that can be created by multiplying elements of an
 /// iterator.
 ///
-/// This trait is used to implement the `product` method on iterators. Types
-/// which implement the trait can be generated by the `product` method. Like
-/// `FromIterator` this trait should rarely be called directly and instead
-/// interacted with through `Iterator::product`.
+/// This trait is used to implement the [`product()`] method on iterators. Types
+/// which implement the trait can be generated by the [`product()`] method. Like
+/// [`FromIterator`] this trait should rarely be called directly and instead
+/// interacted with through [`Iterator::product()`].
+///
+/// [`product()`]: ../../std/iter/trait.Product.html#tymethod.product
+/// [`FromIterator`]: ../../std/iter/trait.FromIterator.html
+/// [`Iterator::product()`]: ../../std/iter/trait.Iterator.html#method.product
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
 pub trait Product<A = Self>: Sized {
     /// Method which takes an iterator and generates `Self` from the elements by