Address CRAN nags on documentation

[eddelbuettel]

This PR rewrites the itemized list in Rd format, re-runs roxygen2 (and addresses the @docType prefers to be replaced by `"_PACKAGE" issue in #20 closing that too) and makes it hopefully easier to get an update with #21 to CRAN. Fingers crossed :) (Any time before CRAN closes, likely in two weeks, would be great).

Anyway, I now see

── R CMD check results ─────────── rncl 0.8.7 ────
Duration: 7.2s

0 errors ✔ | 0 warnings ✔ | 0 notes ✔

so all good here (and that includes the code from #21).

[fmichonneau]

Thanks Dirk!