Tidy get_each_mut documentation and add a changelog entry

Author: Cole Miller

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## Added
 - Added safe `try_insert_no_grow` method to `RawTable`. (#229)
 - Implemented `Default` for `RawTable`. (#237)
+- Added new safe methods `RawTable::get_each_mut`, `HashMap::get_each_mut`, and
+  `HashMap::get_each_key_value_mut`. (#239)
 
 ## Changed
 - The minimum Rust version has been bumped to 1.49.0. (#230)

src/map.rs

@@ -1107,6 +1107,8 @@ where
     ///
     /// This method is available only if the `nightly` feature is enabled.
     ///
+    /// # Examples
+    ///
     /// ```
     /// use hashbrown::{HashMap, UnavailableMutError};
     ///
@@ -1164,6 +1166,8 @@ where
     ///
     /// This method is available only if the `nightly` feature is enabled.
     ///
+    /// # Examples
+    ///
     /// ```
     /// use hashbrown::{HashMap, UnavailableMutError};
     ///