multisig-admin: document the resource-fee cost curve as signer-set size grows, referencing the open benchmark issue

[Idaonoli]

Scope

A resource-fee benchmark for multisig-admin is already tracked (open), but once it lands, the finding still needs to be written up somewhere an issuer sizing their signer set would actually see it — not just left as a passing CI assertion in budget-baselines.toml.

Acceptance criteria

• Depends on the existing open multisig-admin resource-fee benchmark issue landing first
• Once available, add a short guidance section (module doc or a design doc) translating the measured cost curve into a practical recommended maximum signer-set size