Rollup merge of rust-lang#24440 - nham:improve_enumerate_doc, r=alexcrichton

I found that the current description of `enumerate()` doesn't actually tell you what, specifically, the method does, and you have to look at the example to figure it out. Here's a description that I think is better.

Author: steveklabnik

src/libcore/iter.rs

@@ -273,8 +273,9 @@ pub trait Iterator {
         FilterMap { iter: self, f: f }
     }
 
-    /// Creates an iterator that yields a pair of the value returned by this
-    /// iterator plus the current index of iteration.
+    /// Creates an iterator that yields pairs `(i, val)` where `i` is the
+    /// current index of iteration and `val` is the value returned by the
+    /// iterator.
     ///
     /// `enumerate` keeps its count as a `usize`. If you want to count by a
     /// different sized integer, the `zip` function provides similar