Fix doc: start_destroy doesn't need asset to be frozen (#5204)

Fix https://github.com/paritytech/polkadot-sdk/issues/5184

`owner` can set himself as a `freezer` and freeze the asset so
requirement is not really needed. And requirement is not implemented.

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>