GeezerVison

This PR corrects the `views.populate_excel` example and the docs for `UserItem.domain_name` and the user import methods.

• `views.populate_excel`: `excel` is a generator of `bytes` chunks, so the example writes `b''.join(view_item.excel)`. Writing the generator object directly raises `TypeError`.
• `UserItem.domain_name`: the row says what the value means on Tableau Server (the Active Directory domain, or `local`) and on Tableau Cloud (the authentication type). It also says that the property is read-only, so a `UserItem` you create has no domain; include the domain in `name` instead.
• `users.bulk_add`: the example puts the domain in `name` (`example.com\user1`) instead of setting `domain_name`.
• `users.bulk_remove`: the parameter row says each `UserItem` should have `name` and `domain_name` set, as items from `users.get` or `users.filter` do.
• `users.create_from_file`: the whole `Username` value becomes `name`, and `domain_name` is not set, so a domain belongs in the username.

Refs #1128, #1100

Verification: Excel export checked against Tableau Cloud (REST API 3.30) and Tableau Server (REST API 3.31); the output starts with `PK`. `users.get` and `users.get_by_id` populate `domain_name` on Server; checked on 3.31 only.

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