Fix the javadoc for GC.setExpire

Clarify expiration of objects with the modification time exactly at the
given time instant.

Change-Id: I2000aec89c8d6a95700380b0a32275d2d658f67e
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/GC.java

@@ -838,12 +838,15 @@ public class GC {
 
 	/**
 	 * During gc() or prune() each unreferenced, loose object which has been
-	 * created or modified after <code>expire</code> will not be pruned. Only
+	 * created or modified after or at <code>expire</code> will not be pruned.
-	 * older objects may be pruned. If set to null then every object is a
+	 * Only older objects may be pruned. If set to null then every object is a
 	 * candidate for pruning.
 	 *
 	 * @param expire
-	 *            minimal age of objects to be pruned in milliseconds.
+	 *            instant in time which defines object expiration
+	 *            objects with modification time before this instant are expired
+	 *            objects with modification time newer or equal to this instant
+	 *            are not expired
 	 */
 	public void setExpire(Date expire) {
 		this.expire = expire;