Actor.StateManager IActorStateManager is accumulating changes

**What content needs to be created or modified?**
https://docs.dapr.io/developing-applications/sdks/dotnet/dotnet-actors/dotnet-actors-usage/

**Describe the solution you'd like**
Please explicitly specify that Actor.StateManager implements IActorStateManager class accumulates changes during an actor method execution and saves them transactionally in the end of method execution.

**Where should the new material be placed?**
https://docs.dapr.io/developing-applications/sdks/dotnet/dotnet-actors/dotnet-actors-usage/

**The associated pull request from dapr/dapr, dapr/components-contrib, or other Dapr code repos**

**Additional context**
none