Add documentation on poll messages and an outline of Code structure

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133713736

java/google/registry/model/poll/PollMessageExternalKeyConverter.java

@@ -28,7 +28,8 @@ import google.registry.model.reporting.HistoryEntry;
 import java.util.List;
 
 /**
- * A converter between external key strings for PollMessages and Objectify Keys to the resource.
+ * A converter between external key strings for {@link PollMessage}s (i.e. what registrars use to
+ * identify and ACK them) and Datastore keys to the resource.
  *
  * <p>The format of the key string is A-B-C-D-E as follows:
  *