create_database documentation improvements

The [sample code for create_database](https://github.com/googleapis/python-firestore/blob/b8cbf1d32414933f82174a4778d641324c8f1de9/google/cloud/firestore_admin_v1/services/firestore_admin/client.py#L1651) is not correct.

- The documentation says firestore_admin_v1.CreateDatabaseRequest, but it's supposed to be firestore_admin_v1.types.firestore_admin.CreateDatabaseRequest

- The documentation doesn't show using a database param being used, which is required.

- The Database Type is named with an underscore (type_), which is probably generated that way because type is a reserved keyword in Python, perhaps this could be renamed to 'database_type'.

Some example code that I got working is here:

```
import firebase_admin
from firebase_admin import credentials
from firebase_admin import firestore
from google.cloud import firestore
from google.cloud import firestore_admin_v1
from google.cloud.firestore_admin_v1.types import database
from google.cloud.firestore_admin_v1.types import firestore_admin

PROJECT_ID = 'projects/****'

def main(argv):
cred = credentials.ApplicationDefault()
firebase_admin.initialize_app(cred)
admin_client = firestore_admin_v1.FirestoreAdminClient()

db_type = database.Database.DatabaseType.FIRESTORE_NATIVE
db = database.Database(location_id='nam5', type_=db_type)
create_database_request = firestore_admin.CreateDatabaseRequest(
parent=PROJECT_ID, database_id='second-database', database=db
)

create_database_operation = admin_client.create_database(
request=create_database_request
)
print(create_database_operation)
```