Add documentation on command translation. (#41135)

* Add documentation on command translation.

* Restyle

* Rephrased things a bit.

* Restyle

Author: andy31415

docs/guides/migrating_ember_cluster_to_code_driven.md

@@ -105,8 +105,31 @@ new code-driven class structure.
 
 #### Command Handling
 
--   Translate `emberAf...Callback` functions into logic inside your
-    `InvokeCommand` method.
+-   Translate `CommandHandlerInterface` calls or `emberAf...Callback` functions
+    into logic inside your `InvokeCommand` method:
+
+    -   ember calls of the form `emberAf<CLUSTER>Cluster<COMMAND>Callback` will
+        be converted to a switch `case <CLUSTER>::Commands::<COMMAND>::Id: ...`
+        implementation
+
+        Example:
+
+        ```cpp
+        // This
+        emberAfAccessControlClusterReviewFabricRestrictionsCallback(...);
+
+        // Becomes this in the `InvokeCommand` implementation:
+        switch (request.path.mCommandId) {
+          // ...
+          case AccessControl::Commands::ReviewFabricRestrictions::Id:
+             // ...
+        }
+        ```
+
+    -   Command Handler Interface logic translates directly: `CHI` has a switch
+        on command ID inside its `InvokeCommand` call. You should have the same
+        logic inside the `ServerClusterInterface` processing logic.
+
 -   The `InvokeCommand` method can return an `ActionReturnStatus` optional. For
     better readability, prefer returning a status code directly (e.g.,
     `return Status::Success;`) rather than using the command handler to set the