use @noRd to document internal functions

[d-morrison]

following https://style.tidyverse.org/documentation.html#internal-functions

see also https://blog.r-hub.io/2019/12/12/internal-functions/

and https://devguide.ropensci.org/pkg_building.html#:~:text=Add%20%23%27%20%40noRd%20to%20internal%20functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
R/df_to_array.R	0.00% <ø> (ø)

[chrisorwa]

All good.