Clarify what supportsCustomValues in the meta model really means

[DanTup]

I currently have some custom code that determines whether to accept any value for enums, but since the meta model now has supportsCustomValues I planned to migrate to it.

However, it's not clear to me if this value really means what I thought it meant. I thought it meant that if supportsCustomValues != true then the values passed for this enum can be validated (and we can reject a request as having invalid parameters if it passes values that weren't in the spec we used).

However, the SymbolKind enum does not have "supportsCustomValues": true, yet the client can send a DocumentSymbolClientCapabilities that contains a valueSet saying which symbols the client supports, which presumably in the future could be values that my server did not know about when the validation code was written.

So I don't think I can use this field in the way that I hoped, and that makes me wonder what it really means. If we can't assume that !supportsCustomValues means we can validate those values, what can we actually use this flag for?