core::iter::Iterator::flatten: improve docs wrt. deep vs. shallow fla…

…tten per @clarcharr's review

src/libcore/iter/iterator.rs

@@ -1074,6 +1074,26 @@ pub trait Iterator {
     ///                           .collect();
     /// assert_eq!(merged, "alphabetagamma");
     /// ```
+    ///
+    /// Flattening once only removes one level of nesting:
+    ///
+    /// ```
+    /// #![feature(iterator_flatten)]
+    ///
+    /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
+    ///
+    /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
+    /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
+    ///
+    /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
+    /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
+    /// ```
+    ///
+    /// Here we see that `flatten()` does not perform a "deep" flatten.
+    /// Instead, only one level of nesting is removed. That is, if you
+    /// `flatten()` a three-dimensional array the result will be
+    /// two-dimensional and not one-dimensional. To get a one-dimensional
+    /// structure, you have to `flatten()` again.
     #[inline]
     #[unstable(feature = "iterator_flatten", issue = "48213")]
     fn flatten(self) -> Flatten<Self>