Add a couple lines describing differences between into_mut/get_mut.

Author: frewsxcv

src/libstd/collections/hash/map.rs

@@ -2250,6 +2250,11 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
 
     /// Gets a mutable reference to the value in the entry.
     ///
+    /// If you need a reference to the `OccupiedEntry` which may outlive the
+    /// destruction of the `Entry` value, see [`into_mut`].
+    ///
+    /// [`into_mut`]: #method.into_mut
+    ///
     /// # Examples
     ///
     /// ```
@@ -2278,6 +2283,10 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
     /// Converts the OccupiedEntry into a mutable reference to the value in the entry
     /// with a lifetime bound to the map itself.
     ///
+    /// If you need multiple references to the `OccupiedEntry`, see [`get_mut`].
+    ///
+    /// [`get_mut`]: #method.get_mut
+    ///
     /// # Examples
     ///
     /// ```