An article identified by uniq has its NOV data (used
to generate lines in the summary buffer) stored in
.nnmaildir/nov/uniq. There is no
nnmaildir-generate-nov-databases function. (There isn’t much
need for it—an article’s NOV data is updated automatically
when the article or nnmail-extra-headers has changed.) You can
force nnmaildir to regenerate the NOV data for a
single article simply by deleting the corresponding NOV
file, but beware: this will also cause nnmaildir to
assign a new article number for this article, which may cause trouble
with seen marks, the Agent, and the cache.