Expand multisig pallet tests

[rockbmb]

Description

• Add tests to SenderInSignatories/SignatoriesOutOfOrder error cases
• Add clarifying note to approve_as_multi/as_multi comments

Follow-up from open-web3-stack/polkadot-ecosystem-tests#305 and open-web3-stack/polkadot-ecosystem-tests#323

Integration

N/A

Review Notes

N/A

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
• My PR follows the labeling requirements of this project (at minimum one label for T required)
  • External contributors: Use /cmd label <label-name> to add labels
  • Maintainers can also add labels manually
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/21149276373
Failed job name: fmt