Document ComponentUtil.setData

[Legioth]

The ComponentUtil.setData and ComponentUtil.getData API is useful for a wide range of special cases (e.g. to replace custom UI class when updating from an older Vaadin version) but it's not documented anywhere. One particular thing that has confused users is whether the data is stored on the server or in the client.

Could be documented in the Advanced section together with the client-side counterpart: https://github.com/vaadin/docs/blob/latest/articles/flow/advanced/web-storage-api.adoc

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