8288080: (fc) FileChannel::map for MemorySegments should state it always throws UOE

Reviewed-by: alanb

src/java.base/share/classes/java/nio/channels/FileChannel.java

@@ -997,6 +997,9 @@ public abstract class FileChannel
      *           of mapped memory associated with the returned mapped memory
      *           segment is unspecified and should not be relied upon.
      *
+     * @implSpec The default implementation of this method throws
+     *           {@code UnsupportedOperationException}.
+     *
      * @param   mode
      *          The file mapping mode, see
      *          {@link FileChannel#map(FileChannel.MapMode, long, long)};