AnnotationAttributes has existed for several years, but none of the
"get" methods that make up its public API are documented. In many
cases, the behavior can be inferred from the name of the method, but
for some methods there are "hidden gems" and unexpected behavior
lurking behind the scenes.

This commit addresses this issue by documenting all public methods. In
addition, the hidden support for converting single elements into
single-element arrays has also been documented and tested.

Issue: SPR-13072