Dear maintainer,

I have scripts to clean up Debian build products and other unidentified files
outside of VCS-managed directories (e.g. the parent directory in which
Cargo.nail lives, if it's present) and they are complaining that files named
.nailing-cargo.lock are showing up in various places.

I think they are from failed nailing-cargo runs.  It would be good to know
when, and when not, it is okay to delete these files.  Presumably if I know
there are no nailing-cargo compilation processes running then it is fine to
delete them?

Ideally this could be documented, or nailing-cargo could get better at
cleaning them up itself.