Make sum() and product() hyperlinks refer to Iterator methods.

The previous linking seemed confusing: within "the sum() method on
iterators", "sum()" was linked to `Sum::sum`, not `Iterator::sum`, even
though the sentence is talking about the latter.

I have rewritten the sentence to be, I believe, clearer, as well as
changing the link destinations; applying the same change to the
`Product` documentation as well as `Sum`.

library/core/src/iter/traits/accum.rs

@@ -3,12 +3,11 @@ use crate::num::Wrapping;
 
 /// 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 [`Iterator::sum()`]. Types which implement
+/// this trait can be generated by using the [`sum()`] method on an iterator.
+/// Like [`FromIterator`], this trait should rarely be called directly.
 ///
-/// [`sum()`]: Sum::sum
+/// [`sum()`]: Iterator::sum
 /// [`FromIterator`]: iter::FromIterator
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
 pub trait Sum<A = Self>: Sized {
@@ -21,12 +20,11 @@ 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 [`Iterator::product()`]. Types which implement
+/// this trait can be generated by using the [`product()`] method on an iterator.
+/// Like [`FromIterator`], this trait should rarely be called directly.
 ///
-/// [`product()`]: Product::product
+/// [`product()`]: Iterator::product
 /// [`FromIterator`]: iter::FromIterator
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
 pub trait Product<A = Self>: Sized {