Improve javadoc for when to use class names rather than class references

[philwebb]

See https://github.com/spring-projects/spring-framework/issues/35959 for background.

We can improve our class-level Javadoc to indicate when a .class reference can be used and when a String name should be used.

Annotations we might want to consider include @AutoConfigureAfter, @AutoConfigureBefore and @AutoConfiguration.

@ConditionalOnClass has some Javadoc that we could use as a template.