Document issue 21340 - extern(C++, (expression)) now accepts empty tuples

[thewilsonator]

see also dlang/dmd#11905

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21340	enhancement	extern(C++,(emptyTuple)) should result in no namespace not an error

[thewilsonator]

@Geod24 ping for spec PR

[Geod24]

Slight rephrase, oLGTM