Extend Revoke a subkey subsection in adv/signing_components.md with:
Research and explain hardness in the context of subkey revocations. What does a hard subkey revocation express concretely?
Extend `Revoke a subkey` subsection in `adv/signing_components.md` with:
Research and explain hardness in the context of subkey revocations. What does a hard subkey revocation express concretely?