6924219: (fc spec) FileChannel.write(ByteBuffer, position) behavior when file opened for append not specified

Reviewed-by: alanb

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

@@ -146,7 +146,9 @@ import jdk.internal.javac.PreviewFeature;
  * operation first advances the position to the end of the file and then writes
  * the requested data.  Whether the advancement of the position and the writing
  * of the data are done in a single atomic operation is system-dependent and
- * therefore unspecified.
+ * therefore unspecified.  In this mode the behavior of the method to
+ * {@linkplain #write(ByteBuffer,long) write at a given position} is also
+ * system-dependent.
  *
  * @see java.io.FileInputStream#getChannel()
  * @see java.io.FileOutputStream#getChannel()
@@ -193,7 +195,9 @@ public abstract class FileChannel
      *     the position to the end of the file and then writes the requested
      *     data. Whether the advancement of the position and the writing of the
      *     data are done in a single atomic operation is system-dependent and
-     *     therefore unspecified. This option may not be used in conjunction
+     *     therefore unspecified. The effect of {@linkplain
+     *     #write(ByteBuffer,long) writing at a given position} with this option
+     *     present is unspecified. This option may not be used in conjunction
      *     with the {@code READ} or {@code TRUNCATE_EXISTING} options. </td>
      * </tr>
      * <tr>
@@ -809,6 +813,9 @@ public abstract class FileChannel
      * grown to accommodate the new bytes; the values of any bytes between the
      * previous end-of-file and the newly-written bytes are unspecified.  </p>
      *
+     * <p> If the file is open in <a href="#append-mode">append mode</a>, then
+     * the effect of invoking this method is unspecified.
+     *
      * @param  src
      *         The buffer from which bytes are to be transferred
      *