SPIGOT-1356: Clarify expectations about PotionEffectType.values().

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

paper-api/src/main/java/org/bukkit/potion/PotionEffectType.java

@@ -280,6 +280,7 @@ public abstract class PotionEffectType {
 
     /**
      * Returns an array of all the registered {@link PotionEffectType}s.
+     * This array is not necessarily in any particular order and may contain null.
      *
      * @return Array of types.
      */