Add clarifying documentation to Tag

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/Tag.java

@@ -6,6 +6,9 @@ import java.util.Set;
  * Represents a tag that may be defined by the server or a resource pack to
  * group like things together.
  *
+ * Note that whilst all tags defined within this interface must be present in
+ * implementations, their existence is not guaranteed across future versions.
+ *
  * @param <T> the type of things grouped by this tag
  */
 public interface Tag<T extends Keyed> {