docs: Add docs for SIGN_MODE_DIRECT_AUX (#11858)

docs/core/transactions.md

@@ -25,7 +25,7 @@ Transaction objects are Cosmos SDK types that implement the `Tx` interface
 It contains the following methods:
 
 * **GetMsgs:** unwraps the transaction and returns a list of contained `sdk.Msg`s - one transaction may have one or multiple messages, which are defined by module developers.
-* **ValidateBasic:** lightweight, [_stateless_](../basics/tx-lifecycle.md#types-of-checks) checks used by ABCI messages [`CheckTx`](./baseapp.md#checktx) and [`DeliverTx`](./baseapp.md#delivertx) to make sure transactions are not invalid. For example, the [`auth`](https://github.com/cosmos/cosmos-sdk/tree/main/x/auth) module's `StdTx` `ValidateBasic` function checks that its transactions are signed by the correct number of signers and that the fees do not exceed what the user's maximum. Note that this function is to be distinct from `sdk.Msg` [`ValidateBasic`](../basics/tx-lifecycle.md#ValidateBasic) methods, which perform basic validity checks on messages only. When [`runTx`](./baseapp.md#runtx) is checking a transaction created from the [`auth`](https://github.com/cosmos/cosmos-sdk/tree/main/x/auth/spec) module, it first runs `ValidateBasic` on each message, then runs the `auth` module AnteHandler which calls `ValidateBasic` for the transaction itself.
+* **ValidateBasic:** lightweight, [_stateless_](../basics/tx-lifecycle.md#types-of-checks) checks used by ABCI messages [`CheckTx`](./baseapp.md#checktx) and [`DeliverTx`](./baseapp.md#delivertx) to make sure transactions are not invalid. For example, the [`auth`](https://github.com/cosmos/cosmos-sdk/tree/main/x/auth) module's `ValidateBasic` function checks that its transactions are signed by the correct number of signers and that the fees do not exceed what the user's maximum. Note that this function is to be distinct from `sdk.Msg` [`ValidateBasic`](../basics/tx-lifecycle.md#ValidateBasic) methods, which perform basic validity checks on messages only. When [`runTx`](./baseapp.md#runtx) is checking a transaction created from the [`auth`](https://github.com/cosmos/cosmos-sdk/tree/main/x/auth/spec) module, it first runs `ValidateBasic` on each message, then runs the `auth` module AnteHandler which calls `ValidateBasic` for the transaction itself.
 
 As a developer, you should rarely manipulate `Tx` directly, as `Tx` is really an intermediate type used for transaction generation. Instead, developers should prefer the `TxBuilder` interface, which you can learn more about [below](#transaction-generation).
 
@@ -53,13 +53,29 @@ The legacy implemention of the `Tx` interface is the `StdTx` struct from `x/auth
 
 The document signed by all signers is `StdSignDoc`:
 
-+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc3/x/auth/legacy/legacytx/stdsign.go#L20-L33
++++ https://github.com/cosmos/cosmos-sdk/blob/v0.46.0-beta2/x/auth/migrations/legacytx/stdsign.go#L38-L52
 
 which is encoded into bytes using Amino JSON. Once all signatures are gathered into `StdTx`, `StdTx` is serialized using Amino JSON, and these bytes are broadcasted over the network.
 
 #### Other Sign Modes
 
-Other sign modes, most notably `SIGN_MODE_TEXTUAL`, are being discussed. If you wish to learn more about them, please refer to [ADR-020](../architecture/adr-020-protobuf-transaction-encoding.md).
+The Cosmos SDK also provides a couple of other sign modes for particular use cases.
+
+#### `SIGN_MODE_DIRECT_AUX`
+
+`SIGN_MODE_DIRECT_AUX` is a sign mode released in the Cosmos SDK v0.46 which targets transactions with multiple signers. Whereas `SIGN_MODE_DIRECT` expects each signer to sign over both `TxBody` and `AuthInfo` (which includes all other signers' signer infos, i.e. their account sequence, public key and mode info), `SIGN_MODE_DIRECT_AUX` allows N-1 signers to only sign over `TxBody` and _their own_ signer info. Morover, each auxiliary signer (i.e. a signer using `SIGN_MODE_DIRECT_AUX`) doesn't
+need to sign over the fees:
+
++++ https://github.com/cosmos/cosmos-sdk/blob/v0.46.0-beta2/proto/cosmos/tx/v1beta1/tx.proto#L67-L93
+
+The use case is a multi-signer transaction, where one of the signers is appointed to gather all signatures, broadcast the signature and pay for fees, and the others only care about the transaction body. This generally allows for a better multi-signing UX. If Alice, Bob and Charlie are part of a 3-signer transaction, then Alice and Bob can both use `SIGN_MODE_DIRECT_AUX` to sign over the `TxBody` and their own signer info (no need an additional step to gather other signers' ones, like in `SIGN_MODE_DIRECT`), without specifying a fee in their SignDoc. Charlie can then gather both signatures from Alice and Bob, and
+create the final transaction by appending a fee. Note that the fee payer of the transaction (in our case Charlie) must sign over the fees, so must use `SIGN_MODE_DIRECT` or `SIGN_MODE_LEGACY_AMINO_JSON`.
+
+A concrete use case is implemented in [transaction tips](./tips.md): the tipper may use `SIGN_MODE_DIRECT_AUX` to specify a tip in the transaction, without signing over the actual transaction fees. Then, the fee payer appends fees inside the tipper's desired `TxBody`, and as an exchange for paying the fees and broadcasting the transaction, receives the tipper's transaction tips as payment.
+
+#### `SIGN_MODE_TEXTUAL`
+
+`SIGN_MODE_TEXTUAL` is a new sign mode for delivering a better signing experience on hardware wallets, it is currently still under implementation. If you wish to learn more, please refer to [ADR-050](https://github.com/cosmos/cosmos-sdk/pull/10701).
 
 ## Transaction Process