Update thread-safety warning about Repository

Change-Id: I1026a77cc688467d5a89a41121146f1bd3d56fa5

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -106,7 +106,13 @@ import org.slf4j.LoggerFactory;
  * A repository holds all objects and refs used for managing source code (could
  * be any type of file, but source code is what SCM's are typically used for).
  * <p>
- * This class is thread-safe.
+ * The thread-safety of a {@link Repository} very much depends on the concrete
+ * implementation. Applications working with a generic {@code Repository} type
+ * must not assume the instance is thread-safe.
+ * <ul>
+ * <li>{@code FileRepository} is thread-safe.
+ * <li>{@code DfsRepository} thread-safety is determined by its subclass.
+ * </ul>
  */
 public abstract class Repository implements AutoCloseable {
 	private static final Logger LOG = LoggerFactory.getLogger(Repository.class);