`ArtiEphemeralKeystore` docs don't mention swap/suspend-to-disk

I'm opening this as a confidential issue just in case, but I think it's more of a documentation issue than a security issue. Feel free to mark it non-confidential.

tor_keymgr::ArtiEphemeralKeystore is documented with:

This is a purely in-memory key store. Keys written to this store are never written to disk, and are stored in-memory as SshKeyData.

I think it's worth also documenting that they can be written to disk (swap, suspend-to-disk, etc), and that arti doesn't try to prevent this (ex: mlock, memfd_secret maybe?).