Adding more clarity on how sigs are applied to objects.

[jrwds]

Description of the Pull Request (PR):

Just 1 line clarifying why signatures do not have a group.

This fixes or addresses the following GitHub issues:

• Fixes apptainer/apptainer#2159

[DrDaveD]

Thanks! Please make another PR cherry-picking this change to the 1.3 branch

[DrDaveD]

Nevermind, I did the cherry-pick