Warning theme for Badge is not documented

[Dudeplayz]

The warning theme for the Badge is not documented. https://vaadin.com/docs/latest/components/badge

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[mvysny]

Related to https://github.com/vaadin/platform/issues/4074

[HerbertsVaadin]

Updated documentation for Badge to include "warning" variant: https://vaadin.com/docs/latest/components/badge