Add INFER_METHOD constant and update @Bean Javadoc

In anticipation of 'destroy method inference' feature, introduce
ConfigurationClassUtils#INFER_METHOD and update @Bean#destroyMethod to
reflect its use.

Issue: SPR-8751

org.springframework.context/src/main/java/org/springframework/context/annotation/Bean.java

@@ -168,11 +168,21 @@ public @interface Bean {
 	 * The optional name of a method to call on the bean instance upon closing the
 	 * application context, for example a {@code close()} method on a {@code DataSource}.
 	 * The method must have no arguments but may throw any exception.
+	 * <p>As a convenience to the user, the container will attempt to infer a destroy
+	 * method based on the return type of the {@code @Bean} method. For example, given a
+	 * {@code @Bean} method returning an Apache Commons DBCP {@code BasicDataSource}, the
+	 * container will notice the {@code close()} method available on that type and
+	 * automatically register it as the {@code destroyMethod}. By contrast, for a return
+	 * type of JDBC {@code DataSource} interface (which does not declare a {@code close()}
+	 * method, no inference is possible and the user must fall back to manually declaring
+	 * {@code @Bean(destroyMethod="close")}.
+	 * <p>To disable destroy method inference for a particular {@code @Bean}, specify an
+	 * empty string as the value, e.g. {@code @Bean(destroyMethod="")}.
 	 * <p>Note: Only invoked on beans whose lifecycle is under the full control of the
 	 * factory, which is always the case for singletons but not guaranteed 
 	 * for any other scope.
 	 * @see org.springframework.context.ConfigurableApplicationContext#close()
 	 */
-	String destroyMethod() default "";
+	String destroyMethod() default ConfigurationClassUtils.INFER_METHOD;
 
 }

org.springframework.context/src/main/java/org/springframework/context/annotation/ConfigurationClassUtils.java

@@ -46,6 +46,8 @@ abstract class ConfigurationClassUtils {
 	private static final String CONFIGURATION_CLASS_ATTRIBUTE =
 		Conventions.getQualifiedAttributeName(ConfigurationClassPostProcessor.class, "configurationClass");
 
+	static final String INFER_METHOD = ""; // TODO SPR-8751 update to '-' or some such
+
 
 	/**
 	 * Check whether the given bean definition is a candidate for a configuration class,