Update docs for the new added config fixed-array (#1386)

docs/migration-from-v3-to-v4.md

@@ -158,14 +158,19 @@ keep-pec-and-plr: true
 
 ## None-fixed array
 
-In v3, if a response contained a property with an array type, it was generated as a fixed-size C# array that couldn't support adding new elements through functions like Add. In v4, we generate a list instead of an array to support the Add function in such cases.
+In v3, if a response contained a property with an array type, it was generated as a fixed-size C# array that couldn't support adding new elements through functions like Add. In v4, by default, we generate a list instead of an array to support the Add function in such cases. As a result, from users' perspective, the following modifications should be made in scripts that utilize PowerShell modules generated with v4:
+
+- Use Count instead of Length to retrieve the number of elements
+- Use the ToArray function if conversion to a fixed-size array is needed
 
 ### How to mitigate the breaking changes of non-fixed array
 
-This is a design change and we do not provide a way to generate fixed array anymore. From users' perspective, the following modifications should be made in scripts that utilize PowerShell modules generated with v4:
+Configure `fixed-array` as `true` in README.md
+
+```
+fixed-array: true
+```
 
-- Use Count instead of Length to retrieve the number of elements
-- Use the ToArray function if conversion to a fixed-size array is needed
 
 ## X-ms-mutability support