docs: added a services constraint document (#8999)

- Added a services constraint document mentioning that a service's methods must be async.
- Modify the code in the services intro document to use `async` method.

www/apps/book/app/advanced-development/modules/service-constraints/page.mdx

@@ -0,0 +1,34 @@
+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 adds wrappers around your service's methods and executes them as async methods.
+
+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/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
+```

www/apps/book/app/basics/modules-and-services/page.mdx

@@ -38,7 +38,7 @@ For example, create the file `src/modules/hello/service.ts` with the following c
 
 ```ts title="src/modules/hello/service.ts"
 export default class HelloModuleService {
-  getMessage() {
+  async getMessage() {
     return "Hello, world!"
   }
 }
@@ -121,7 +121,7 @@ export async function GET(
   )
 
   res.json({
-    message: helloModuleService.getMessage(),
+    message: await helloModuleService.getMessage(),
   })
 }
 ```

www/apps/book/generated/edit-dates.mjs

@@ -3,7 +3,7 @@ export const generatedEditDates = {
   "app/basics/workflows/page.mdx": "2024-09-03T08:07:16.276Z",
   "app/deployment/page.mdx": "2024-08-05T07:24:05+00:00",
   "app/page.mdx": "2024-09-03T07:09:09.034Z",
-  "app/basics/modules-and-services/page.mdx": "2024-09-03T07:45:28.079Z",
+  "app/basics/modules-and-services/page.mdx": "2024-09-04T15:37:27.695Z",
   "app/basics/commerce-modules/page.mdx": "2024-09-03T07:48:48.148Z",
   "app/advanced-development/workflows/retry-failed-steps/page.mdx": "2024-07-31T17:01:33+03:00",
   "app/advanced-development/workflows/workflow-hooks/page.mdx": "2024-09-10T11:39:51.168Z",
@@ -72,6 +72,7 @@ export const generatedEditDates = {
   "app/debugging-and-testing/testing-tools/page.mdx": "2024-09-10T11:39:51.172Z",
   "app/debugging-and-testing/testing-tools/unit-tests/module-example/page.mdx": "2024-09-02T11:04:27.232Z",
   "app/debugging-and-testing/testing-tools/unit-tests/page.mdx": "2024-09-02T11:03:26.997Z",
+  "app/advanced-development/modules/service-constraints/page.mdx": "2024-09-04T15:37:04.166Z",
   "app/advanced-development/api-routes/page.mdx": "2024-09-04T09:36:33.961Z",
   "app/advanced-development/api-routes/responses/page.mdx": "2024-09-10T11:39:51.167Z",
   "app/advanced-development/api-routes/validation/page.mdx": "2024-09-10T11:39:51.167Z",

www/apps/book/sidebar.mjs

@@ -150,6 +150,11 @@ export const sidebar = numberSidebarItems(
               path: "/advanced-development/modules/service-factory",
               title: "Service Factory",
             },
+            {
+              type: "link",
+              path: "/advanced-development/modules/service-constraints",
+              title: "Service Constraints",
+            },
             {
               type: "link",
               path: "/advanced-development/modules/isolation",