docs: fix upgrade guide (#114)
docs: fix upgrade guide (#114)

Current upgrade guide is confusing especially when upgrading from v2.0.0 to v3.0.0 because some of the change is only introduced at v2.0.0. I fixed this.

This is just a documentation fix rather than code fix, so note that following checks are not passed

---

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Fixes #<issue_number_goes_here> 🦕

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## Additional question

There are three way to invoke API-calling methods: (1) pass keyword arguments, (2) pass `request` argument with request class and (3) pass `request` argument with dict.

The auto-migration tool `fixup_datacatalog_v1_keywords.py` fixes code to use method (3) even if the methods accepts keyword arguments. However, IMO, it's poorly typed and not linter/autocompletion-friendly.

Actually what is the recommended way? And is there a plan to remove support of method (1)?

| | Available since | Typed | Note |
|:---|:---|:---|:---|
| (1) | v1 | ◎ (since v2) | |
| (2) | v2 | △ | Code looks redundant |
| (3) | v2 | ☓ | |

(1)

```py
response = client.create_entry_group(
    parent=parent, 
    entry_group_id=entry_group_id,
    entry_group=entry_group
)
```

(2)

```py
response = client.create_entry_group(
    datacatalog.CreateEntryGroupRequest(
        parent=parent, 
        entry_group_id=entry_group_id,
        entry_group=entry_group
    )
)
```

(3)

```py
response = client.create_entry_group(
    request={
        "parent": parent,
        "entry_group_id": entry_group_id,
        "entry_group": entry_group
    }
)
```


googleapis/python-datacatalog,googleapis/python-datacatalog