Update actor serialization documentation

**What content needs to be created or modified?**
While the documentation at [https://github.com/dapr/dotnet-sdk/blob/master/daprdocs/content/en/dotnet-sdk-docs/dotnet-actors/dotnet-actors-serialization.md](https://github.com/dapr/dotnet-sdk/blob/master/daprdocs/content/en/dotnet-sdk-docs/dotnet-actors/dotnet-actors-serialization.md) accurately describes the strong-typed version of the actor serialization, it could do more to indicate that JSON serialization is used actors referenced via the weakly-typed factory.

**Where should the new material be placed?**
Because of the length of the current document, it might make sense to write a separate document for the JSON serialization in actors and then still another that has some sort of flow chart and description that gives better guidance as to when a developer should opt for either approach.

**The associated pull request from dapr/dapr, dapr/components-contrib, or other Dapr code repos**
Not necessarily applicable, though the dotnet-sdk Actor DI registration methods could be improved on to reflect the updated documentation in the XML comments to provide richer in-place documentation.

**Additional context**
Would close the following issues:
https://github.com/dapr/dotnet-sdk/issues/1050
https://github.com/dapr/dotnet-sdk/issues/1138
https://github.com/dapr/dotnet-sdk/issues/891