Improve `in_parallel()` docs from carrier

[shikokuchuo]

This carrier commit: r-lib/carrier@b58c6a3 by @lionel- contains very clear examples of how to use crate for functions which require helper functions and other parameters.

purrr::in_parallel() is just a thin wrapper around carrier::crate().

Some of the documentation for in_parallel() can benefit from being expanded to make usage clearer, such as explicitly showing both helper functions and parameters.