Extend Sequence Location documentation to discuss optional sequence reference

[ahwagner]

SequenceLocation should require either sequenceReference or sequence to be populated

No. Sequence is just descriptive of the content described by the SequenceLocation, it is not definitive like a sequenceReference is. SequenceReference is optional due to its use in CisPhasedBlock Alleles. We recognize that the documentation needs to be extended to discuss this nuance and we are taking that issue on in #564.

Originally posted by @ahwagner in #533 (reply in thread)