docs: clarify SNS ledger fee parameter updates

[bjoernek]

Summary

Clarifies the SNS Governance protobuf/API comments around ledger fee updates:

• ManageLedgerParameters.transfer_fee is the intended way to change the SNS ledger's actual transfer fee.
• Successful ManageLedgerParameters execution also syncs NervousSystemParameters.transaction_fee_e8s in Governance.
• Directly changing NervousSystemParameters.transaction_fee_e8s does not update the ledger.

Why

DAO operators can otherwise infer that changing transaction_fee_e8s through ManageNervousSystemParameters changes the ledger fee. That can leave Governance and the SNS ledger fee out of sync, causing ordinary ledger transfers initiated by Governance to use a stale fee.

Validation

• git diff --check
• rustfmt --check rs/sns/governance/src/gen/ic_sns_governance.pb.v1.rs

No runtime tests were run because this is a comment-only documentation change.

[daniel-wong-dfinity-org-twin]

Ideally, we'd get rid of this footgun, but ofc, that would require more effort, which we do not want to spend now.

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

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Brief Guide to "Externally Visible" Changes

"Externally visible behavior change" is very often due to some NEW canister API.

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If these changes are breaking, make sure that clients know how to migrate, how to
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Reference(s)

For a more comprehensive checklist, see here.

GOVERNANCE_CHECKLIST_REMINDER_DEDUP

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to df0ab23.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f975f1a5-0a47-4a3b-b7f0-a55609e90390?scanId=7716407f-81dc-44fa-9c23-560f357d9f9f&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/sns/governance/proto/ic_sns_governance/pb/v1/governance.proto     A proposal function that changes the SNS ledger canister's parameters. ► rs/sns/governance/src/gen/ic_sns_governance.pb.v1.rs     A proposal function that changes the SNS ledger canister's parameters.
Enhancement	► rs/sns/governance/proto/ic_sns_governance/pb/v1/governance.proto     ManageLedgerParameters: add transfer_fee field with detailed comments ► rs/sns/governance/src/gen/ic_sns_governance.pb.v1.rs     ManageLedgerParameters: add transfer_fee field with detailed comments ► rs/sns/governance/proto/ic_sns_governance/pb/v1/governance.proto     NervousSystemParameters: add transaction_fee_e8s field description aligned with governance behavior ► rs/sns/governance/src/gen/ic_sns_governance.pb.v1.rs     NervousSystemParameters: add transaction_fee_e8s field description aligned with governance behavior