Note that I’ve put ^+ after \mathrm{Normal}^+ to show that it only takes positive values.

I’m a bit uncertain how \tau should be displayed, but I guess the above will let you explain it to the reader, i.e., as @paul.buerkner writes in the documentation ?set_prior:

each spline has its corresponding standard deviations modeling the variability within this term

I recommend looking at James Hodges’s work. The link above mentions a paper of his on random effects, but there is a lot more on his website that would help with notation, including course materials that go along with his book, Richly Parameterized Linear Models.