docs: clarify how to change the SNS token transfer fee (#306)

## Summary

A forum discussion surfaced a common point of confusion: changing
`transaction_fee_e8s` via a `ManageNervousSystemParameters` proposal
only updates Governance's stored parameter and does **not** change the
actual SNS ledger transfer fee. The native path is a
`ManageLedgerParameters` proposal with `transfer_fee` set, which updates
the ledger and syncs Governance's `transaction_fee_e8s` automatically.
Confirmed by the SNS team (Björn).

This PR makes that distinction explicit across all three places the
parameter appears:

- **`docs/guides/governance/managing.md`** — adds a `:::caution` under
`ManageNervousSystemParameters` (right after the
`NervousSystemParameters` Candid type) warning against using
`transaction_fee_e8s` to change the fee, and pointing to
`ManageLedgerParameters`.
- **`docs/guides/governance/managing.md`** — adds a sentence to the
`ManageLedgerParameters` section noting that `transfer_fee` is the field
to use and that it syncs Governance automatically, so no separate
proposal is needed.
- **`docs/references/sns-settings.md`** — rewords the
`transaction_fee_e8s` reference row, which previously described it as
the "per-transfer fee on the SNS ledger" (the misleading framing), to
clarify it is Governance's stored copy and that the fee is changed via
`ManageLedgerParameters`.

`npm run build` passes.

---------

Co-authored-by: Bjoern Assmann <bjoern.assmann@dfinity.org>

Author: marc0olo

docs/guides/governance/managing.md

@@ -121,6 +121,12 @@ type NervousSystemParameters = record {
 };
 ```
 
+:::caution[Changing the SNS token transfer fee]
+Do not use `ManageNervousSystemParameters.transaction_fee_e8s` to change the SNS token transfer fee. This field updates only Governance's stored parameter, not the SNS ledger canister. Worse, because Governance uses this stored value as the fee for its own neuron-operation transfers (disbursing, splitting, staking maturity, and so on), setting it to a value that differs from the ledger's actual fee will cause those transfers to be rejected by the ledger (`BadFee`), breaking neuron operations.
+
+To change the actual ledger transfer fee, submit a [`ManageLedgerParameters`](#manageledgerparameters) proposal with `transfer_fee` set. On successful execution, the ledger fee is updated and Governance's `transaction_fee_e8s` is synced to the same value automatically.
+:::
+
 For a description of each parameter and its effect, see the [SNS settings reference](../../references/sns-settings.md).
 
 ### ManageSnsMetadata
@@ -155,6 +161,8 @@ quill send message.json
 
 Updates ledger parameters: transfer fee, token name, token symbol, or token logo. Fields set to `null` remain unchanged.
 
+Use `transfer_fee` here to change the SNS token transfer fee; this is the only proposal that updates the actual fee charged by the ledger. On successful execution, it also syncs Governance's `NervousSystemParameters.transaction_fee_e8s` to the same value, so a separate `ManageNervousSystemParameters` proposal is not needed.
+
 ```bash
 quill sns \
   make-proposal $PROPOSAL_NEURON_ID \

docs/references/sns-settings.md

@@ -43,7 +43,7 @@ For background, see [SNS framework](../concepts/sns-framework.md).
 
 | Parameter | Type | Description |
 |---|---|---|
-| `transaction_fee_e8s` | `nat64` | Per-transfer fee on the SNS ledger, in e8s. Does not apply to minting or burning. |
+| `transaction_fee_e8s` | `nat64` | Governance's stored copy of the per-transfer ledger fee, in e8s. Does not apply to minting or burning. Do not set this field via `ManageNervousSystemParameters` to change the fee: it updates only Governance's copy, not the ledger. Change the fee with a `ManageLedgerParameters` proposal instead, which updates the ledger and syncs this field automatically (see [Managing an SNS](../guides/governance/managing.md#manageledgerparameters)). |
 
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