Document suppression of kableExtra printing in Rmd/etc. documents

Inspired by #145. Should probably consider whether there's a better way to do it, such as the way this user attempted to do it before filing the issue.

No file, test, or entry point is named in the issue. Start by reading issue #145 and the earlier user attempt it references, then determine what suppression behavior for kableExtra in Rmd and similar documents should be documented; done means the relevant usage and preferred approach are clearly described.

Written by the indexing model from the issue text.