docs: revise main docs outline (#10502)

www/apps/book/app/learn/fundamentals/modules/service-constraints/page.mdx

@@ -0,0 +1,40 @@
+export const metadata = {
+  title: `${pageNumber} Service Constraints`,
+}
+
+# {metadata.title}
+
+This chapter lists constraints to keep in mind when creating a service.
+
+## Use Async Methods
+
+Medusa wraps service method executions to inject useful context or transactions. However, since Medusa can't detect whether the method is asynchronous, it always executes methods in the wrapper with the `await` keyword.
+
+For example, if you have a synchronous `getMessage` method, and you use it in other resources like workflows, Medusa executes it as an async method:
+
+```ts
+await helloModuleService.getMessage()
+```
+
+So, make sure your service's methods are always async to avoid unexpected errors or behavior.
+
+```ts highlights={[["8", "", "Method must be async."], ["13", "async", "Correct way of defining the method."]]}
+import { MedusaService } from "@medusajs/framework/utils"
+import MyCustom from "./models/my-custom"
+
+class HelloModuleService extends MedusaService({
+  MyCustom,
+}){
+  // Don't
+  getMessage(): string {
+    return "Hello, World!"
+  }
+
+  // Do
+  async getMessage(): Promise<string> {
+    return "Hello, World!"
+  }
+}
+
+export default HelloModuleService
+```