Add documentation to UserAttributeSubpacketTags

Author: vanitasvitae

pg/src/main/java/org/bouncycastle/bcpg/UserAttributeSubpacketTags.java

@@ -2,8 +2,16 @@
 
 /**
  * Basic PGP user attribute sub-packet tag types.
+ *
+ * @see <a href="https://datatracker.ietf.org/doc/html/rfc4880#section-5.12">
+ *     RFC4880 - User Attribute Packet</a>
+ * @see <a href="https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-13.html#name-user-attribute-packet-type-">
+ *     C-R - User Attribute Packet</a>
  */
 public interface UserAttributeSubpacketTags 
 {
+    /**
+     * Tag for an {@link org.bouncycastle.bcpg.attr.ImageAttribute}.
+     */
     int IMAGE_ATTRIBUTE = 1;
 }