Smaller fixups for "versioned" struct/docs and clarifications

When implementing things for NumPy, I noticed a few things:
* All other structs are also `typedef`'d, the new `...Versioned` is not.
* In our discussions at some point we had a large comment in place for how far `DLTensorVersioned` is ABI stable. This is missing (i.e. we want everything up to and including the flags to be ABI stable, after that all bets are off for major versions).
* When the major version changes, we may need a new struct. I am wondering if it might still make sense to just rip the band-aid off and *remove* `dl_tensor` from the `...Versioned` struct and introduce a `...V1` which _includes_ it.
That would make it abundantly clear that you need to cast. If v2 doesn't need a new struct, could make it an alias or just document that?

On the Python side, `copy=True` and espeically `copy=None` behavior could be clarified. I did the lazy path in NumPy and just always copied with `copy=True`.
In very rare cases that might make an array exportable, which isn't without `copy=True` (due to odd strides).

It may make sense to clarify that `copy=None` is *only* required to do copies to make device requests work (if they are supported).