Allow schema.TypeMap to have schema.Resource Elem type.

### SDK version

```
v2.0.4
```

### Use-cases
Attributes of resource or data source can then be used directly in another resources for_each.

### Attempted Solutions
Explicitly convert returned list to a map in for_each expression

### Proposal
Allow schema.TypeMap to have schema.Resource Elem type and update documentation

### References
https://github.com/hashicorp/terraform-plugin-sdk/issues/601

Possible duplicate of https://github.com/hashicorp/terraform-plugin-sdk/issues/616

Start by locating the schema.TypeMap and schema.Resource handling in the SDK, then read referenced issues 601 and 616 for existing design context. Review the related tests and documentation once found; done means Resource elements are accepted for TypeMap and the behavior is documented and covered by tests.

Written by the indexing model from the issue text.