Document @testinstance lifecycle support in the M5 Release Notes

Issue: #419

Author: sbrannen

documentation/src/docs/asciidoc/release-notes-5.0.0-M5.adoc

@@ -3,8 +3,8 @@
 
 *Date of Release:* ❓
 
-*Scope:* Fifth milestone release of JUnit 5 with a focus on dynamic containers and minor
-API changes.
+*Scope:* Fifth milestone release of JUnit 5 with a focus on dynamic containers, test
+instance lifecycle management, and minor API changes.
 
 WARNING: This is a milestone release and contains breaking changes. Please refer to the
 <<running-tests-ide-intellij-idea,instructions>> above to use this version in a version of
@@ -38,6 +38,7 @@ is placed on the Java 9 module path.
 | `junit-vintage-engine-<VERSION>.jar`              | `org.junit.vintage.engine`
 |===
 
+
 [[release-notes-5.0.0-m5-junit-platform]]
 ==== JUnit Platform
 
@@ -69,7 +70,7 @@ is placed on the Java 9 module path.
   - `junit-platform-commons`: `ReflectionUtils.findAllClassesInClasspathRoot(Path, Predicate, Predicate)`
 * The `isLeaf()` method of the `org.junit.platform.engine.support.hierarchical.Node`
   interface has been removed.
-* The default methods `pruneTree()`, and `hasTests()` have been removed from
+* The default methods `pruneTree()` and `hasTests()` have been removed from
   `TestDescriptor`.
 
 ===== New Features and Improvements
@@ -94,6 +95,7 @@ is placed on the Java 9 module path.
 * The `ENGINE` enum constant was removed from `TestDescriptor.Type`. The default type of
   the `EngineDescriptor` is now `TestDescriptor.Type.CONTAINER`.
 
+
 [[release-notes-5.0.0-m5-junit-jupiter]]
 ==== JUnit Jupiter
 
@@ -116,6 +118,9 @@ is placed on the Java 9 module path.
   the functionality for creating an `Arguments` instance is now available via the
   `Arguments.of(...)` static factory method.
 * The `names` property of `@MethodSource` has been renamed to `value`.
+* The `getTestInstance()` method in the `TestExtensionContext` API has been moved to the
+  `ExtensionContext` API. Furthermore, the signature has changed from
+  `Object getTestInstance()` to `Optional<Object> getTestInstance()`.
 
 [[release-notes-5.0.0-m5-migration-extension-api]]
 .Extension API Migration
@@ -133,6 +138,18 @@ is placed on the Java 9 module path.
 
 ===== New Features and Improvements
 
+* The test instance lifecycle can now be switched from the default per-method mode to a
+  new per-class mode via the new class-level `@TestInstance` annotation. This enables
+  shared test instance state between test methods in a given test class as well as
+  between non-static `@BeforeAll` and `@AfterAll` methods in the test class.
+  - See <<writing-tests-test-instance-lifecycle>> for details.
+* `@BeforeAll` and `@AfterAll` methods are no longer required to be `static` if the test
+  class is annotated with `@TestInstance(Lifecycle.PER_CLASS)`. This enables the
+  following new features.
+  - Declaration of `@BeforeAll` and `@AfterAll` methods in `@Nested` test classes.
+  - Declaration of `@BeforeAll` and `@AfterAll` on interface `default` methods.
+  - Simplified declaration of `@BeforeAll` and `@AfterAll` methods in test classes
+    implemented with the Kotlin programming language.
 * `Assertions.assertAll()` now tracks exceptions of any type (as opposed to only tracking
   exceptions of type `AssertionError`), unless the exception is a _blacklisted_ exception
   in which case it will be immediately rethrown.
@@ -149,6 +166,7 @@ is placed on the Java 9 module path.
 * `@TestFactory` now supports arbitrarily nested dynamic containers. See `DynamicContainer`
   and abstract base class `DynamicNode` for details.
 
+
 [[release-notes-5.0.0-m5-junit-vintage]]
 ==== JUnit Vintage