fix: roxygen2 warnings and improve doc formatting

[salim-b]

Fixes all roxygen2 warnings and improves documentation formatting at some places. See the individual commits for details.

Some more useful info on @exporting S3 methods is found in this comment by Garrick Aden-Buie: https://github.com/r-lib/roxygen2/issues/1592#issuecomment-1986069247