Add javadoc on RevCommit

Change-Id: Ib413154fe52983286cb6307862a7373af4fec6e8
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
index 7f1e887..a7c21e3 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
@@ -36,6 +36,11 @@
 
 /**
  * A commit reference to a commit in the DAG.
+ *
+ * The state of the RevCommit isn't populated until the commit is parsed. The
+ * newly created RevCommit is unparsed and only has an objectId reference. Other
+ * states like parents, trees, commit ident, commit message, etc. are
+ * populated/available when the commit is parsed.
  */
 public class RevCommit extends RevObject {
 	private static final int STACK_DEPTH = 500;