Summary
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`MappedByteBuffer` does not specify that bulk copying between overlapping mapped regions of the same file has unspecified results.

Problem
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If two MappedByteBuffers are created from overlapping ranges of the same file and a bulk copy is performed between them, then surprising and unexpected results may occur.

Solution
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Add verbiage stating that that bulk copying between MappedByteBuffers mapped from overlapping ranges of the same file has unspecified results.

Specification
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Add this sentence before the current last sentence of the class level specification of `MappedByteBuffer`:

```The result of a bulk copy between two {@code MappedByteBuffer}s is
unspecified if they are mapped from overlapping regions of the same file.```

A webrev is attached.