Document `@AliasFor` support in `AnnotatedTypeMetadata.getAnnotationAttributes()`

[sbrannen]

While working on #31041, I encountered a pleasant surprise: AnnotatedTypeMetadata.getAnnotationAttributes() actually supports full @AliasFor semantics.

The Javadoc for getAnnotationAttributes() has historically stated that it supported "attribute overrides on composed annotations"; however, it turns out that it actually supports @AliasFor in general, including attribute aliases within a given annotation.

This is likely a result of the switch to the MergedAnnotations API in Spring Framework 5.2.

We should therefore document that.