ChangeData: add Javadoc

Change-Id: Ifcbbf53b8f30f90d8928c35b78d39a9bb5e28f94

java/com/google/gerrit/server/query/change/ChangeData.java

diff --git a/java/com/google/gerrit/server/query/change/ChangeData.java b/java/com/google/gerrit/server/query/change/ChangeData.java
index 6de263e..3c877e5 100644
--- a/java/com/google/gerrit/server/query/change/ChangeData.java
+++ b/java/com/google/gerrit/server/query/change/ChangeData.java
@@ -103,6 +103,13 @@
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevWalk;
 
+/**
+ * ChangeData provides lazily loaded interface to change metadata loaded from NoteDb. It can be
+ * constructed by loading from NoteDb, or calling setters. The latter happens when ChangeData is
+ * retrieved through the change index. This happens for Applications that are performance sensitive
+ * (eg. dashboard loads, git protocol negotiation) but can tolerate staleness. In that case, setting
+ * lazyLoad=false disables loading from NoteDb, so we don't accidentally enable a slow path.
+ */
 public class ChangeData {
   public static List<Change> asChanges(List<ChangeData> changeDatas) {
     List<Change> result = new ArrayList<>(changeDatas.size());