Tom Dorner wrote to me to share what he observed while he has been getting acquainted with Revit 2016. It's a new Type of Parameter called Multiline Text.
When you see it in the Properties Palette it offers up a small Browse button. Tom commented that he thought it would be better if it was an Edit button like the one that Model Text use, I'm inclined to agree.
Clicking the browse button opens the Edit Text dialog.
I'm still waiting for my download to finish so I can get it installed myself and check it out more closely. I'm curious how it will behave if the parameter is used in a schedule or tag. I haven't seen any evidence that Autodesk is taking credit for this addition in the What's New information they've made available so far.