clarify evaluation result expectation

[markus-metzger]

https://github.com/ccoutant/dwarf-locations/blame/b1d75746e53e1bf090e1c112a7a157f59bf1c550/002-clarifications-eval.txt#L38 says

When a DWARF expression is evaluated, it may be
specified whether a value or location description is required as the
result kind.

I think it should say it must be specified, since we can otherwise not distinguish a value from a memory location description.

[t-tye]

There are a couple of places in DWARF 5 where the result can be either a value or a location description, with special rules that happen depending on what the evaluation ends up returning. It would be nice to eliminate these cases, but this proposal did not do that to preserve backwards compatibility