Document that annotations are ignored if attributes reference types not present in the classpath

[sbrannen]

Overview

Annotation search algorithms in Spring Framework typically ignore annotations that cannot be loaded because annotation attributes reference types that are not present in the classpath.

Sometimes these scenarios are logged at WARN, INFO, or DEBUG level; however, we don't actually document when or where this can occur.

We should therefore update the Javadoc to point out such scenarios — for example, in MergedAnnotations, AnnotationMetadata, etc.

Related Issues

• Log warning when meta-annotation is ignored due to types not present in classpath #35927
• Improve javadoc for when to use class names rather than class references spring-boot#48395