Document new PackWriter#preparePack variant's parameters and exceptions

Change-Id: Id4fa272c611a855bf4ef1bf5399f3e4305664103

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

@@ -723,6 +723,26 @@ public class PackWriter implements AutoCloseable {
 	 * be shallow any more ("unshallow" commits as in {@link #setShallowPack})
 	 * from the shallow set.
 	 *
+	 * @param countingMonitor
+	 *            progress during object enumeration.
+	 * @param want
+	 *            objects of interest, ancestors of which will be included in
+	 *            the pack. Must not be {@code null}.
+	 * @param have
+	 *            objects whose ancestors (up to and including
+	 *            {@code shallow} commits) do not need to be included in the
+	 *            pack because they are already available from elsewhere.
+	 *            Must not be {@code null}.
+	 * @param shallow
+	 *            commits indicating the boundary of the history marked with
+	 *            {@code have}. Shallow commits have parents but those
+	 *            parents are considered not to be already available.
+	 *            Parents of {@code shallow} commits and earlier generations
+	 *            will be included in the pack if requested by {@code want}.
+	 *            Must not be {@code null}.
+	 * @param IOException
+	 *            an I/O problem occured while reading objects.
+	 *
 	 * @since 4.5
 	 */
 	public void preparePack(ProgressMonitor countingMonitor,