docs: document TreeGrid.getDataProvider without list/lazy views (vaadin/flow#19376)

[AzazelSensei]

Description

TreeGrid inherits Grid.getDataProvider javadoc, which points at getListDataView() and getLazyDataView(). Both throw on TreeGrid. I added javadoc on the override so the API docs describe the hierarchical provider instead.

Fixes vaadin/flow#19376

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}