Improve Cached annotation documentation

Author: bdonlan

src/main/java/com/github/rschmitt/dynamicobject/Cached.java

@@ -9,6 +9,12 @@
  * Requests that DynamicObject serializers with support for caching or deduplicating repeated values cache the value
  * stored under the associated getter or builder. Currently, only the Fressian serializer makes use of this annotation.
  * <p/>
+ * The objects stored under this annotation should implement a sensible equals() and hashCode(); additionally, when
+ * using @Cached, you should be cognizant that the cache used by the serializer may be quite small - Fressian has only
+ * 32 entries, for instance, and this is shared with cached map keys as well. Therefore, using @Cached on data that
+ * cannot be effectively cached can greatly negatively impact the size of the encoded data. In particular, using @Cached
+ * on data that uses default object identity comparison may be a very bad idea.
+ * <p/>
  * This annotation does not impact binary compatibility of the serialized data.
  *
  * @since 1.6.0