Document for the pulsar.Result enum

The `pulsar.Result` enum is just imported from the C extension. See https://github.com/apache/pulsar-client-python/blob/daabc677e06e867f84beaeb3912d1d450cddf54d/pulsar/__init__.py#L50

Running `help(pulsar.Result)` could only show the underlying C error codes like

```
|
| AlreadyClosed =
|
| AuthenticationError =
|
| AuthorizationError =
|
| BrokerMetadataError =
|
| BrokerPersistenceError =
```

The error code should be well documented. Otherwise users might not know how to handle these error codes.

For now, users can refer https://github.com/apache/pulsar-client-cpp/blob/main/include/pulsar/Result.h for the meaning of error codes.