doclines per fn non-exported does not count correctly

[mpadge]

ropensci/software-review#715 (comment) from @ScottClaessens is indeed correct; calling pkgstats() on https://github.com/ScottClaessens/coevolve says doclines_per_fn_non_exported = 0, even though there are doclines for internal fns.

[mpadge]

Fixed by #82, which added https://github.com/ropensci-review-tools/pkgstats/blob/main/R/roxygen-blocks.R. This only gets docline stats for non-exported functions from roxygen2, and so still won't count other ways to document internal functions other than that. Need to ensure that non-roxygen approaches to docs generate NA values and not 0, so that CRAN-wide statistics can be extracted only from the subset of packages using roxgyen2.