add a description to JUnit launcher in plugin.xml

bundles/com.salesforce.bazel.eclipse.core/plugin.xml

@@ -165,6 +165,7 @@
       <launchDelegate
             name="Bazel JUnit"
             modes="run, debug"
+            delegateDescription="The Bazel JUnit Launcher *must* be used for launching JUnit tests in a Bazel workspace. See the Bazel Eclipse Feature launcher documentation for more details."
             type="org.eclipse.jdt.junit.launchconfig"
             delegate="com.salesforce.bazel.eclipse.launch.BazelJunitLaunchConfigurationDelegate"
             id="bazel.junit.launcher">

bundles/com.salesforce.bazel.eclipse.core/src/main/java/com/salesforce/bazel/eclipse/launch/BazelJunitLaunchConfigurationDelegate.java

@@ -44,13 +44,19 @@
 /**
  * Plug point into the Eclipse JUnit launcher call chain; allows us to inject the Bazel runtime classpath provider so
  * the test uses the Bazel computed classpath.
+ * <p>
+ * Note that the user must elect to use this delegate (referred to as the 'Launcher' in the UI). This delegate is
+ * registered in plugin.xml as a "org.eclipse.jdt.junit.launchconfig" type delegate. This means it will compete with the
+ * native Eclipse delegate for JUnit launching duties. The user will be presented with a UI for them to choose the
+ * delegate they want, and they *must* choose the Bazel one.
  */
 public class BazelJunitLaunchConfigurationDelegate extends JUnitLaunchConfigurationDelegate {
 
     @Override
     public void launch(ILaunchConfiguration configuration, String mode, ILaunch launch, IProgressMonitor monitor)
             throws CoreException {
 
+        // the enable method sets an attribute on the configuration to use the Bazel test classpath provider
         BazelTestClasspathProvider.enable(configuration);
         BazelTestClasspathProvider.canOpenErrorDialog.set(true);