[Python] Add doc comment for WriteToText.skip_if_empty

[riteshghorse]

Fixes #27926

Add clarifying doc comment for skip_if_empty in WriteToText

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[damccorm]

I don't think we should actually change the behavior here. I'd vote we just document the parameter a little better. I left a couple comments in the issue this is responding to.

[riteshghorse]

That makes sense. I think it will confuse users if the file exists even though PCollection was empty.