Update comments about PackStatistics

Change-Id: I4862932dc38b58cc98400ed584cc506013846065
Signed-off-by: Terry Parker <tparker@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java

@@ -1032,8 +1032,8 @@ public class PackWriter implements AutoCloseable {
 
 	/**
 	 * @return description of what this PackWriter did in order to create the
-	 *         final pack stream. The object is only available to callers after
-	 *         {@link #writePack(ProgressMonitor, ProgressMonitor, OutputStream)}
+	 *         final pack stream. This should only be invoked after the calls to
+	 *         create the pack/index/bitmap have completed.
 	 */
 	public PackStatistics getStatistics() {
 		return new PackStatistics(stats);

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackStatistics.java

@@ -101,6 +101,12 @@ public class PackStatistics {
 		 *            the accumulator of the statistics
 		 */
 		public ObjectType(ObjectType.Accumulator accumulator) {
+			/*
+			 * For efficiency this wraps and serves up the Accumulator object
+			 * rather than making a deep clone. Normal usage of PackWriter is to
+			 * create a single pack/index/bitmap and only call getStatistics()
+			 * after all work is complete.
+			 */
 			objectType = accumulator;
 		}
 
@@ -251,8 +257,12 @@ public class PackStatistics {
 	 *            the accumulator of the statistics
 	 */
 	public PackStatistics(Accumulator accumulator) {
-		// Note: PackStatistics directly serves up the collections in the
-		// accumulator.
+		/*
+		 * For efficiency this wraps and serves up the Accumulator object rather
+		 * than making a deep clone. Normal usage of PackWriter is to create a
+		 * single pack/index/bitmap and only call getStatistics() after all work
+		 * is complete.
+		 */
 		statistics = accumulator;
 	}