Document SetAssigneeSender

Change-Id: I0d74e841626cf86ed02efee001534a08a8343d38
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/java/com/google/gerrit/server/mail/send/SetAssigneeSender.java b/java/com/google/gerrit/server/mail/send/SetAssigneeSender.java
index 850f775..5f370a0 100644
--- a/java/com/google/gerrit/server/mail/send/SetAssigneeSender.java
+++ b/java/com/google/gerrit/server/mail/send/SetAssigneeSender.java
@@ -22,6 +22,13 @@
 import com.google.inject.Inject;
 import com.google.inject.assistedinject.Assisted;
 
+/**
+ * Sender that informs a user by email that they were set as assignee on a change.
+ *
+ * <p>In contrast to other change emails this email is not sent to the change authors (owner, patch
+ * set uploader, author). This is why this class extends {@link ChangeEmail} directly, instead of
+ * extending {@link ReplyToChangeSender}.
+ */
 public class SetAssigneeSender extends ChangeEmail {
   public interface Factory {
     SetAssigneeSender create(Project.NameKey project, Change.Id changeId, Account.Id assignee);