docs: Update checkout lib migration doc (#12811)

closes GH-12808

docs/migration/4_0.md

@@ -289,14 +289,40 @@ Lib: @spartacus/core
 Class: UserAddressService
 Change: Two new required constructor parameters `userAddressConnector: UserAddressConnector` and `command: CommandService`
 
+
+## New Checkout Library
+
+Spartacus 4.0 introduces the checkout library.  The checkout related code is moved out of `@spartacus/core` and `@spartacus/storefrontlib` into one of the checkout lib's entry points.  The checkout library is split into these entry points:
+
+```
+@spartacus/checkout/assets 
+( checkout related i18n keys are moved here )
+
+@spartacus/checkout/components
+( checkout related UI codee is moved here. This includes components, guards and ui services )
+
+@spartacus/checkout/core
+The checkout facade API implementation are moved here, as well as connectors, event builder, event listener, models, other services, and state management.
+
+@spartacus/checkout/occ
+The checkout related OCC code is moved here. This includes the checkout related adapters and converters.
+
+@spartacus/checkout/root
+The root entry point is, by convention, meant to always be eager loaded.  It contains the config, events, facades, http interceptors and models.
+
+@spartacus/checkout/styles
+The checkout related scss styles are moved here.
+
+```
+
+Most of the code is moved unchanged, but some classes required changes after they were moved.  See the section below for the list:
+
 ## (start) Changes in the classes carried over to the @spartacus/checkout lib 
 
 ### CheckoutEventModule
-Class: CheckoutEventModule
 Change: One new required constructor parameters `_checkoutEventListener: CheckoutEventListener`
 
 ### PaymentFormComponent
-Class: PaymentFormComponent
 Changes: 
 - PaymentFormComponent does not implement `OnDestroy` anymore
 - method `ngOnDestroy()` removed.