docs: improvements + additions to module docs (#9152)

- Split Module and Module Links to their own chapters
- Add new docs on db operations and transactions in modules, multiple services, links with custom columns, etc...
- Added a list of registered dependencies in a module container

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

@@ -8,7 +8,13 @@ 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.
+Medusa wraps service method executions to inject useful context or transactions. However, since Medusa can't detect whether the method is asynchronus, it always executes methods in the wrapper with the `await` keyword.
+
+For example, if you have a synchronous `getMessage` method, and you use it 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.