docs(store): document new bevahior of on

[timdeschryver]

Other information:

With #2103 merged, we have to change the documentation.

Before the behavior was that if an action was registered twice via the on function in createReducer, the last one would override the previous ones.

The new behavior allows multiple on functions for the same action, in the same createReducer.

I could only find one occurrence of it at:

platform/modules/store/src/reducer_creator.ts

Lines 204 to 208 in 9a70262

	* - Must be used with `ActionCreator`'s (returned by `createAction`). Cannot be used with class-based action creators.
	* - An action type should only be associated with at most one state change function, similar to switch statements.
	* - In the case this is violated, the latest defined associated will be used (the latest `on` function passed).
	* - The returned `ActionReducer` should additionally be returned from an exported `reducer` function.
	* This is because [function calls are not supported](https://angular.io/guide/aot-compiler#function-calls-are-not-supported) by the AOT compiler.