discuss threads in importing vignette

[tdhock]

@grantmcdermott suggested improving the importing vignette, by adding a discussion about how to avoid check issues about using too many threads. #5658 (comment)

I would suggest something like:

Add data.table::setDTthreads(1) to the top of your tests/*.R files and also at the top of the \examples section in man/*.Rd files.

[grantmcdermott]

Thanks @tdhock.

I think adding some nuance for different use cases (examples vs tests vs vignettes) would be helpful. See Jan's original comment here. E.g. For Examples, better to wrap in \dontshow{data.table::setDTthreads(2)}.

[aitap]

Maybe not only \dontshow{setDTthreads(...)} at the beginning, but also \dontshow{setDTthreads(.previously.saved.value); rm(.previously.saved.value)} at the end to minimise the changes to the global state.