[#654] Explain absence of ToStorageType TAddress instance

Description

Problem: TAddress isn't intended to be a part of Cleveland API, but it is extensively used in older code. This can create an incentive to add a ToStorageType (TAddress a) instance as a quick fix. However, there isn't a valid use case for such an instance.

Solution: Add a Haddock comment explaining why there is no such instance and alternatives.

Related issue(s)

Resolves #654 (closed)

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