Document memory improvements in release notes

documentation/src/docs/asciidoc/release-notes/release-notes-5.8.0-M1.adoc

@@ -39,6 +39,8 @@ on GitHub.
 * Custom `LauncherDiscoveryListener` implementations can now be registered via Java’s
   `ServiceLoader` mechanism.
 * Documented constant value of `ExclusiveResource.GLOBAL_KEY`.
+* Instances of `TestIdentifier` and `UniqueId` now retain less memory because they no
+  longer store `String` representations of unique IDs.
 
 
 [[release-notes-5.8.0-M1-junit-jupiter]]
@@ -60,6 +62,8 @@ on GitHub.
 * Documented constant values in `Resources`.
 * New `assertInstanceOf` methods as a replacement for `assertTrue(obj instanceof X)` which
   provide better error messages comparable to those of `assertThrows`.
+* Dynamic tests now require less memory thanks to a number of improvements to internal
+  data structures.
 
 [[release-notes-5.8.0-M1-junit-vintage]]
 === JUnit Vintage
@@ -74,4 +78,5 @@ on GitHub.
 
 ==== New Features and Improvements
 
-* ❓
+* The JUnit Vintage engine now requires less memory and allows for earlier garbage
+  collection thanks to a number of improvements to internal data structures.