Polish "Polish GradleCompatibilityExtension's javadoc"

See gh-17411

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/junit/GradleCompatibilityExtension.java

@@ -20,6 +20,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.stream.Stream;
 
+import org.junit.jupiter.api.TestTemplate;
 import org.junit.jupiter.api.extension.Extension;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.TestTemplateInvocationContext;
@@ -29,9 +30,9 @@ import org.springframework.boot.gradle.testkit.GradleBuild;
 import org.springframework.boot.gradle.testkit.GradleBuildExtension;
 
 /**
- * {@link Extension} that runs {@link org.junit.jupiter.api.TestTemplate templated tests}
+ * {@link Extension} that runs {@link TestTemplate templated tests} against multiple
- * against multiple versions of Gradle. Test classes using the extension must have a
+ * versions of Gradle. Test classes using the extension must have a non-private and
- * non-private and non-final {@link GradleBuild} field named {@code gradleBuild}.
+ * non-final {@link GradleBuild} field named {@code gradleBuild}.
  *
  * @author Andy Wilkinson
  */