Update docs with description tags where needed

[sjspielman]

It turns out that the "description" sections for roxygen docs don't get fully rendered if there are line paragraph breaks but there is no literal @description tag: https://cran.r-project.org/web/packages/roxygen2/vignettes/rd.html#the-introduction

This definitely affects a couple functions in scpcaTools, so we should go through functions to make sure all the docs end up as part of the docs.

Edit:
Back to note that this wasn't exactly correct - docs are all present, but the ones following paragraph breaks are just in a different docs section, which isn't strictly a problem!

Therefore, we should instead ensure that our docs follow the R standard for docs organization, as described in AlexsLemonade/rOpenScPCA#26