Update proxyTargetClass-related Javadoc; add tests

There was some question about whether enabling subclass proxies via
proxyTargetClass / proxy-target-class settings would break annotation-
based demarcation of joinpoints due to inability to discover those
annotations in various scenarios. The provided tests prove that in
any conceivable case, these annotations (@Transactional, at least)
are discovered in a consistent fashion, meaning that switching proxy
strategies should be transparent to the application and honor
intended annotation semantics.

org.springframework.context/src/main/java/org/springframework/scheduling/annotation/EnableAsync.java

@@ -125,9 +125,12 @@ public @interface EnableAsync {
 	 * to standard Java interface-based proxies. The default is {@code false}. <strong>
 	 * Applicable only if {@link #mode()} is set to {@link AdviceMode#PROXY}</strong>.
 	 *
-	 * <p>Note that subclass-based proxies require the async {@link #annotation()}
-	 * to be defined on the concrete class. Annotations in interfaces will
-	 * not work in that case (they will rather only work with interface-based proxies)!
+	 * <p>Note that setting this attribute to {@code true} will affect <em>all</em>
+	 * Spring-managed beans requiring proxying, not just those marked with {@code @Async}.
+	 * For example, other beans marked with Spring's {@code @Transactional} annotation
+	 * will be upgraded to subclass proxying at the same time. This approach has no
+	 * negative impact in practice unless one is explicitly expecting one type of proxy
+	 * vs another, e.g. in tests.
 	 */
 	boolean proxyTargetClass() default false;