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

Summary

Add an @implSpec annotation to the method

java.nio.channels.FileChannel.map(FileChannel.MapMode,long,long,MemorySession)

for creating mapped memory segments indicating that it always throw an UnsupportedOperationException.

Problem

FileChannel.map() for mapped memory segments always throws UnsupportedOperationException but this is not explicitly stated.

Solution

Add an @implSpec annotation to the method which states that an UnsupportedOperationException is always thrown.

Specification

--- a/src/java.base/share/classes/java/nio/channels/FileChannel.java
+++ b/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)};