This is actually a mistake in the documentation, and offset and maxreturn are not valid parameters. Thanks for bringing this to my attention, I will correct the doc.
I would also add that maxReturn is not working for retrieving all segmentations. There are always 20 segmentations shown despite my maxReturn=100 parameter. (/rest/asset/v1/segmentation.json?maxReturn=100)
Link to documentation: Get Segmentations » Marketo Developers
What am I doing wrong ?