RFR: 8288080: (fc) FileChannel::map for MemorySegments should state it always throws UOE [v2]

[Brian Burkhalter]

On Thu, 9 Jun 2022 08:39:05 GMT, Uwe Schindler <uschindler at openjdk.org> wrote:

>> Alternative: "The default implementation of `map(MapMode, long, long, MemorySession)` method in abstract `FileChannel` always throws an `UnsupportedOperationException`. Subclasses should override it."

I simplified it to match similar statements in `Selector` and `SelectorProvider`, for example. I don't think that the second sentence about overriding is needed.

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PR: https://git.openjdk.org/jdk/pull/9095