Document 'annotated classes' in Testing chapter

In order to help developers avoid surprises from unintended use of
"@Bean Lite Mode" when referencing classes via @ContextConfiguration in
the TestContext framework, this commit duplicates the previous
discussion of "annotated classes" from the Javadoc for
@ContextConfiguration as a tip in the "Context configuration with
annotated classes" section of the Testing chapter in the reference
manual.

Issue: SPR-10971