Clarify that a for loop uses IntoIterator

Author: shepmaster

src/doc/book/loops.md

@@ -80,13 +80,15 @@ for var in expression {
 }
 ```
 
-The expression is an [iterator][iterator]. The iterator gives back a series of
-elements. Each element is one iteration of the loop. That value is then bound
-to the name `var`, which is valid for the loop body. Once the body is over, the
-next value is fetched from the iterator, and we loop another time. When there
-are no more values, the `for` loop is over.
+The expression is an item can can be converted into an [iterator] using
+[`IntoIterator`]. The iterator gives back a series of elements. Each element is
+one iteration of the loop. That value is then bound to the name `var`, which is
+valid for the loop body. Once the body is over, the next value is fetched from
+the iterator, and we loop another time. When there are no more values, the `for`
+loop is over.
 
 [iterator]: iterators.html
+[`IntoIterator`]: ../std/iter/trait.IntoIterator.html
 
 In our example, `0..10` is an expression that takes a start and an end position,
 and gives an iterator over those values. The upper bound is exclusive, though,