(A reminder for myself as much as anything...)

efisiglist will only build a sig list by adding and removing
certificates, It's often useful (to me!) to be able to convert back
from a sig list to a set of certificates, which it can't do.

However, sig-list-to-certs in the efitools package knows how to do
this. Let's add a mention of that in the efisiglist man page!