confusingly worded warning when tspan doesn't include root

[ajwheeler]

The warning introduced in #158 is great, however the first time I got it I had to do some googling to figure out what it meant. Specifically the wording made me think that opposite signs were required for tspan[1] and tspan[2]. I suggest tweaking it to say that tspan doesn't include a root, or similar.

I can open a PR if you approve of this change.

[ChrisRackauckas]

Yeah, open a PR. It's easiest to understand whether the new wording is better by seeing the suggestion.