docs: how to specify any, union or variant type for a schema

[christophediprima]

Please describe the issue

I would like to define a schema like this with a column being of any or variant type but cant find a way to do it.
The only one that was accepted by ibis but not by Flink is following:

ibis.schema({"variant-field": "unknown"})

Flink as ANY but not UNKNOWN

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