Document that `@ConditionalOnClass` should not be used directly on methods

The condition can't be put in places where the JVM may have loaded the class and look at method definitions. 

A typical use case is a `@Bean` method that returns something and that something may not be on the classpath. Using the condition on the method will be useless as the JVM will load the class before the condition kicks in.

It would be nice to update both the ref guide and the javadoc.