docs(transactions.service): update TODO comments for transactions

Refine and expand TODO comments to outline specific tasks for managing transactions. Detail steps for handling single/multiple products and listing transactions for users and products.

src/transactions/transactions.service.ts

@@ -2,5 +2,16 @@ import { Injectable } from '@nestjs/common';
 
 @Injectable()
 export class TransactionsService {
-  //TODO CRUD of transactions
+  //TODO List transaction of a user
+  //TODO List transaction related to a product
+
+  //TODO Transaction for multiple/single products by a user
+    //Check stocks
+    //Check account credit
+      //take the money
+      //Update stock
+      //update history
+      //send response, refurnish if error
+
+  //TODO List all transaction
 }