Document that `attribute` is optional in `@⁠AliasFor` for overrides with the same name

[sbrannen]

Originally, the attribute attribute in @AliasFor was required; however, we later made it optional when the aliasing attribute and the aliased attribute (in a meta-annotation) have the same name.

For example, the path attribute in @GetMapping omits the attribute = "path" declaration, since the annotation = RequestMapping.class is sufficient.

/**
 * Alias for {@link RequestMapping#path}.
 */
@AliasFor(annotation = RequestMapping.class)
String[] path() default {};

We should therefore document this feature in the Javadoc for @AliasFor to make users aware of this.