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

[Brian Burkhalter]

Add an `@implSpec` annotation to the `java.nio.channels.FileChannel::map` method for mapped memory segments indicating that it always throws an `UnsupportedOperationException`.

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

Changes: https://git.openjdk.java.net/jdk/pull/9095/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9095&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288080
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9095.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9095/head:pull/9095

PR: https://git.openjdk.java.net/jdk/pull/9095