Segment Features: "Extend" and other features are lacking documentation

[imagejan]

This is related to #449, but also true for the (Image) Segment Features nodes: documentation about the features is essentially absent. You have to dig in the source code to find how it's computed.

For example, the "Extend" feature is probably a mis-nomer and should read "Extent"? I don't know if it's documented anywhere, but it looks like it corresponds to the Matlab implementation explained here:
https://ch.mathworks.com/help/images/ref/regionprops.html#buoixjn-1-properties