Clarify the difference between get_mut and into_mut for OccupiedEntry

The examples for both hash_map::OccupiedEntry::get_mut and hash_map::OccupiedEntry::into_mut were almost identical. This led to some confusion over the difference, namely why you would ever use get_mut when into_mut gives alonger lifetime. Reddit thread: https://www.reddit.com/r/rust/comments/8a5swr/why_does_hashmaps This commit adds two lines and a comment to the example, to show that the entry object can be re-used after calling get_mut.

Author: Phlosioneer

src/libstd/collections/hash/map.rs

@@ -2261,10 +2261,14 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
  ///
  /// assert_eq!(map["poneyland"], 12);
  /// if let Entry::Occupied(mut o) = map.entry("poneyland") {
- /// *o.get_mut() += 10;
+ /// *o.get_mut() += 10;
+ /// assert_eq!(o.get(), 22);
+ ///
+ /// // We can use the same Entry multiple times.
+ /// *o.get_mut() += 2;
  /// }
  ///
- /// assert_eq!(map["poneyland"], 22);
+ /// assert_eq!(map["poneyland"], 24);
  /// ```
  #[stable(feature = "rust1", since = "1.0.0")]
  pub fn get_mut(&mut self) -> &mut V {