Improve docs on what the skan measurements actually mean

[GenevieveBuckley]

Juan says here that the docs in skan need to be improved, to prevent misunderstanding like the one I made here: https://forum.image.sc/t/how-to-dilate-skeleton-based-on-vessel-diamater/65549/4?

It would be good to improve the docs for all the statistical metrics skan produces.
But the specific measurement I'd confused was:

• branch-distance: the total length of the branch. Eg: a string laid along the branch, touching it all the way)
• euclidean-distance: the straight line distance between the start and end points of the branch. Eg: a string pulled taught between the branch start and end. This is (almost) always smaller than the branch-distance (it will be equal to the branch distance in cases where the branch is entirely straight)

[GenevieveBuckley]

I confused euclidean-distance with the thickness of a branch, probably because the name made me think of euclidean distance transforms, and assumed that was used to measure the distance away from the skeleton centreline.

Juan says that actually nothing in skan measures branch thickness at all.

Currently, the branch thickness is not stored anywhere in skan. There are workarounds that I have been meaning to turn into a tutorial and/or build into skan properly, but aren’t there yet.