RFR: 8286735: (fc) FileChannel#map should document behavior when using custom file systems

[Brian Burkhalter]

Clarify when the `map()` methods of `java.nio.channels.FileChannel` throw an `UnsupportedOperationException`.

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Commit messages:
 - 8286735: (fc) FileChannel#map should document behavior when using custom file systems

Changes: https://git.openjdk.java.net/jdk/pull/8739/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8739&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286735
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8739.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8739/head:pull/8739

PR: https://git.openjdk.java.net/jdk/pull/8739