Initial treasury docs (#5678)

* initial treasury docs

* add info

* Update guide

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Filippo <[email protected]>

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Filippo <[email protected]>

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Filippo <[email protected]>

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Filippo <[email protected]>

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Filippo <[email protected]>

* address feedback

* slight change

* change to spendLocal

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Radha <[email protected]>

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Radha <[email protected]>

* Update docs/learn/learn-polkadot-opengov-treasury.md

Co-authored-by: Radha <[email protected]>

* -> point not rate

---------

Co-authored-by: Filippo <[email protected]>
Co-authored-by: Radha <[email protected]>

docs/learn/learn-guides-polkadot-opengov.md

@@ -52,7 +52,7 @@ Follow the steps below to submit a preimage as shown in the screenshot above.
 2. Click on the "Add preimage" button.
 3. From the _propose_ drop-down field, select `treasury`.
 4. From the unlabeled drop-down field to the right of the _propose_ drop-down field, select
-   `spend(amount, beneficiary)`.
+   `spendLocal(amount, beneficiary)`.
 5. In the `amount: Compact<u128> (BalanceOf)` text field, enter the spend amount.
 6. The `beneficiary: MultiAddress (AccountIdLookupOf)` drop-down field will have `Id` selected by
    default. Select the beneficiary from the `Id: AccountId` drop-down field.

docs/learn/learn-guides-treasury.md

@@ -64,7 +64,7 @@ that requests 100 DOT from Treasury.
 - Navigate to [Polkadot-JS UI > Governance > Preimages](https://polkadot.js.org/apps/#/preimages)
   and then click on Add Preimage.
 - Select the account which will be used to submit the preimage.
-- Choose `treasury` pallet in the "propose" dropdown and the `spend(amount, beneficiary)`call
+- Choose `treasury` pallet in the "propose" dropdown and the `spend_local(amount, beneficiary)`call
 - Enter the DOT amount.
 - Enter the AccountID of the beneficiary (which has a verified on-chain identity).
 - Submit preimage
@@ -127,8 +127,8 @@ to the `32 / Small Spender` track, you will need to submit it to the `30 / Small
 
 Briefly, you will need to:
 
-- Create a preimage using the `treasury.Spend` extrinsic and specifying the number of tokens and the
-  beneficiary of the tip
+- Create a preimage using the `treasury.spendLocal` extrinsic and specifying the number of tokens
+  and the beneficiary of the tip
 - Submit a proposal to the right track (i.e. `30` or `31`) using the preimage hash
 - Once you started the referendum go to [Polkassembly](https://polkassembly.io/), log in with the
   proposer account and edit the referendum details

docs/learn/learn-polkadot-opengov-treasury.md

@@ -10,7 +10,7 @@ slug: ../learn-polkadot-opengov-treasury
 import RPC from "./../../components/RPC-Connection";
 
 The Treasury is a pot of funds collected through a portion of block production rewards, transaction
-fees, slashing, and [staking inefficiencies](./learn-inflation.md).Treasury funds are held in a
+fees, slashing, and [staking inefficiencies](./learn-inflation.md). Treasury funds are held in a
 [system account](./learn-account-advanced.md#system-accounts) that cannot be controlled by any
 external account; only the system internal logic can access it.
 
@@ -98,6 +98,40 @@ bottom right corner. Then,
   - Create a proposal: final confirmation about the proposal creation. The description of the
     proposal and the preimage are automatically linked to the proposal.
 
+## Sub-treasuries
+
+The {{ polkadot: Polkadot :polkadot }}{{ kusama: Kusama :kusama }} treasury currently operates on
+a single account on-chain. The above tracks manage the outflow of the treasury on the
+network. With _sub_-treasuries, having treasury accounts that correspond to each
+[collective](./learn-system-chains#collectives) is also possible.
+
+Rather than have many referenda through OpenGov, the
+{{ polkadot: Polkadot :polkadot }}{{ kusama: Kusama :kusama }} Treasury can allocate funds to each
+sub-treasury (through [governance](./learn-polkadot-opengov)), from which each respective collective
+can spend funds (depending on their specific rule set).
+
+New treasuries could be added to respective
+[system chains](./learn-system-chains#existing-system-chains) through governance by adding more
+instances of this pallet.
+
+## Multi-Asset Treasury Support
+
+The treasuries can support multiple asset types and thus can spend assets other than
+{{ polkadot: DOT :polkadot }}{{ kusama: KSM :kusama }} held within the treasury, and their transfers and interactions across the chains facilitated by [cross-consensus messaging](./learn-xcm.md). These assets have a
+few requirements:
+
+1. The asset is listed on the [AssetHub system parachain](https://assethub-polkadot.subscan.io/).
+2. The asset is active and has sufficient liquidity to be utilized for payouts.
+3. The asset has a set conversion rate, as per OpenGov referenda on the Treasurer track (set via the
+   asset rate pallet). This conversion rate defines a fixed-point representation for converting from
+   that asset to {{ polkadot: DOT :polkadot }}{{ kusama: KSM :kusama }}.
+4. The asset must be approved and onboarded via OpenGov to become spendable via the treasury as a
+   valid spend method.
+
+> For example,
+> [see how USDT became approved as an asset on AssetHub](https://forum.polkadot.network/t/multi-asset-treasury-and-milestone-based-spends/6780),
+> which can be used in the treasury.
+
 ## Bounties
 
 ### Parent Bounties