Feature Request: Document Cloud Storage doesn't validate the existence of logging_bucket in gsutil logging set on command

https://github.com/GoogleCloudPlatform/gsutil/blob/master/gslib/commands/logging.py#L57

From:

```
logging_bucket must already exist, and all URLs must name
buckets (e.g., gs://bucket).
```

to:

```
Cloud Storage doesn't validate the existence of logging_bucket so users should ensure it already exists, and all URLs must name buckets (e.g., gs://bucket).
```

Start at gslib/commands/logging.py around line 57, where the logging_bucket guidance is documented. Update the wording to explain that Cloud Storage does not validate the bucket's existence and that users should ensure it exists, while preserving the URL requirement. Done means the command documentation matches the requested text.

Written by the indexing model from the issue text.