Why the range U+0320–U+03FF when computing spacing?

This is separated out from #167 since the other issues are settled and it should be closed for CR.

Core says:

If Content is a single character in the range U+0320–U+03FF then exit with category Default.

That ranges makes no sense to me. It covers part of the combining chars and also the Greek/Coptic chars. I think maybe it is trying to capture the combining chars, but the combining chars range is U+0300 - U+036F. There are additional combining chars 1AB0–1AFF and 1DC0–1DFF that maybe should be included.

And from a later comment:

I still don't see why U+0320–U+03FF makes sense. Why are some combining chars included in the range and not others? Why is a Greek alpha treated different than a latin a? Although you (@fred-wang) don't need include text in the spec why this is so, it seems like a bug to me so you should explain why it isn't a bug.

Start with the Core rule quoted in the issue and verify U+0320–U+03FF against the Unicode combining-character ranges cited. Done when the range is justified in the specification or the rule is corrected and the issue’s concern is resolved.