docs(*): deal with change in function selector names

The old format is `namespace_system_function`.
The new format is `namespace__function`.

This should be merged at the same time as #2160, which implements the change.

docs/pages/guides/extending-world.mdx

@@ -375,7 +375,7 @@ The easiest way to create the Solidity code is to use the MUD template:
 
    Register `MessageSystem.incrementMessage(string)`.
    This step is necessary to make the function accessible through the `World`.
-   The function's name when accessed through the world is `<namespace>_<system>_<function>`, so this function will be available as `messaging_message_incrementMessage(string)`.
+   The function's name when accessed through the world is `<namespace>__<function>`, so this function will be available as `messaging__incrementMessage(string)`.
 
    **Note:** This is the case for all namespaces except for the root namespace, where we just use the name of the function (such as `increment()`).
 
@@ -401,10 +401,10 @@ The easiest way to create the Solidity code is to use the MUD template:
 
    ```sh copy
    source .env
-   cast send $WORLD_ADDRESS --rpc-url http://localhost:8545 --private-key $PRIVATE_KEY "messaging_MessageSystem_incrementMessage(string)" "hello"
+   cast send $WORLD_ADDRESS --rpc-url http://localhost:8545 --private-key $PRIVATE_KEY "messaging__incrementMessage(string)" "hello"
    ```
 
-   When a function is _not_ in the root namespace, it is accessible as `<namespace>_<system>_<function name>` (as long as it is [registered](https://github.com/latticexyz/mud/blob/main/packages/world/src/modules/core/implementations/WorldRegistrationSystem.sol#L164-L201)).
+   When a function is _not_ in the root namespace, it is accessible as `<namespace>__<function name>` (as long as it is [registered](https://github.com/latticexyz/mud/blob/main/packages/world/src/modules/core/implementations/WorldRegistrationSystem.sol#L164-L201)).
 
    Here we call our `incrementMessage(string)` with the parameter `hello`.
 
@@ -561,7 +561,7 @@ We will fix these problems now.
 
 ### Increment and send a message
 
-Edit `src/mud/createSystemCalls.ts` to use the new `messaging_MessageSystem_incrementMessage` function instead of `increment`.
+Edit `src/mud/createSystemCalls.ts` to use the new `messaging__incrementMessage` function instead of `increment`.
 
    <CollapseCode>
 
@@ -608,7 +608,7 @@ Edit `src/mud/createSystemCalls.ts` to use the new `messaging_MessageSystem_incr
         * is in the root namespace, `.increment` can be called directly
         * on the World contract.
         */
-          const tx = await worldContract.write.messaging_MessageSystem_incrementMessage(
+          const tx = await worldContract.write.messaging__incrementMessage(
               [`message at ${Date()}`]);
           await waitForTransaction(tx);
           return getComponentValue(Counter, singletonEntity);

docs/pages/world/function-selectors.mdx

@@ -12,7 +12,7 @@ If [you have the necessary delegation](/world/account-delegation) you can also u
 
 To improve the developer experience of calling `System` functions, the namespace owner can register a function selector for each `System` function in the `World` contract.
 This removes the need for the caller to know the system's `ResourceId`, and the need to manually encode the calldata via `abi.encodeCall`.
-Once the namespace owner registers a function selector for a `System` function, anybody can call the `System` via `<World>.<namespace>_<system>_<function>(<args>)`.
+Once the namespace owner registers a function selector for a `System` function, anybody can call the `System` via `<World>.<namespace>__<function>(<args>)`.
 
 To register a function selector, you use [`<world>.registerFunctionSelector`](https://github.com/latticexyz/mud/blob/main/packages/world/src/modules/core/implementations/WorldRegistrationSystem.sol#L164-L201).
 This function takes two parameters:
@@ -34,7 +34,7 @@ This is the code we use to register the `System` and the function selector:
 After the function selector is registered, it can be accessed directly on the World:
 
 ```solidity
-    world.messaging_message_incrementMessage("hello");
+    world.messaging__incrementMessage("hello");
 ```
 
 ## Root function selectors

docs/pages/world/systems.mdx

@@ -20,7 +20,7 @@ The `World` serves as a central entry point and forwards calls to systems, which
 
 To call a `System`, you call the `World` in one of these ways:
 
-- If a [function selector for the `System` is registered in the `World`](./function-selectors), you can call it via `world.<namespace>_<system>_<function>(<arguments>)`.
+- If a [function selector for the `System` is registered in the `World`](./function-selectors), you can call it via `world.<namespace>__<function>(<arguments>)`.
 - You can use [`call`](https://github.com/latticexyz/mud/blob/main/packages/world/src/World.sol#L346-L351).
 - If you have [the proper delegation](/world/account-delegation) you can use [`callFrom`](https://github.com/latticexyz/mud/blob/main/packages/world/src/World.sol#L353-L394).
 

docs/pages/world/world-table-illustration.mdx

@@ -1,7 +1,7 @@
 ![Interaction between the `World`, a `System`, and a table](./world-table.svg)
 
-1. An account calls a function called `namespace_system_function` via the `World`.
-   [This function was registered](./function-selectors) by the owner of the `namespace` namespace and points to the `function` function in the `system` system in the `namespace` namespace.
+1. An account calls a function called `namespace__function` via the `World`.
+   [This function was registered](./function-selectors) by the owner of the `namespace` namespace and points to the `function` function in one of the `System`s in the `namespace` namespace.
 
 2. The `World` verifies that access is permitted (for example, because `namespace:system` is publicly accessible) and if so calls `function` on the `namespace:system` contract with the provided parameters.