(bf) MappedByteBuffer.force method may have no effect on implementation specific map modes

Summary

Update MappedByteBuffer.force javadoc to encompass implementtaion-specific map modes

Problem

JDK-8221397 allowed for a MappedByteBuffer to be mapped using an implementation-specific map mode. The javadoc for force was not updated to explain that a request may complete with no effect in such a case.

Solution

Update the javadoc to include this outcome.

Specification

@@ -232,13 +232,13 @@
      * invoked, will have been written to that device.
      *
      * <p> If the file does not reside on a local device then no such guarantee
      * is made.
      *
-     * <p> If this buffer was not mapped in read/write mode ({@link
-     * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
-     * method has no effect. </p>
+     * <p> This method has no effect for buffers mapped in read-only
+     * or private mapping modes. It may also have no effect for other
+     * implementation specific map modes. </p>
      *
      * @return  This buffer
      */
     public final MappedByteBuffer force() {
         if (fd == null) {

@@ -267,13 +267,13 @@
      * are transferred in their entirety.
      *
      * <p> If the file does not reside on a local device then no such
      * guarantee is made.
      *
-     * <p> If this buffer was not mapped in read/write mode ({@link
-     * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then
-     * invoking this method has no effect. </p>
+     * <p> This method has no effect for buffers mapped in read-only
+     * or private mapping modes. It may also have no effect for other
+     * implementation specific map modes. </p>
      *
      * @param index
      *        The index of the first byte in the buffer region that is
      *        to be written back to storage; must be non-negative
      *        and less than limit()