8312491: Update Classfile API snippets and examples

Reviewed-by: jlahoda

src/java.base/share/classes/jdk/internal/classfile/Attribute.java

@@ -72,6 +72,7 @@ import jdk.internal.classfile.impl.UnboundAttribute;
  * corresponding model type. Additionally, all attributes are accessible
  * directly from the corresponding model type through {@link
  * AttributedElement#findAttribute(AttributeMapper)}.
+ * @param <A> the attribute type
  */
 public sealed interface Attribute<A extends Attribute<A>>
         extends WritableElement<A>