Update variationReference.avdl

We've clarified some language and asked some questions where we are unsure.
mostly we have vendor-specific ids, but there are some exceptions. 
`reference.ncbiTaxonId` is one example of a global namespace (which is maintained by ncbi) and the `reference.md5checksum` could be used as an ID (there was discussion on this)

specifically for the allele ID we can definitely come up with a plan for globalness (and in that case we could have a non-string type). There just has to be a clear set of rules for how the IDs are made/maintained and what not. 

So I guess my string comment was just referring to the general sense. I wasn't clear on who was owning GAAllele :)