ADR-017: clarify IsShelleyBasedEra vs type families distinction

[Copilot]

The type families section conflated two distinct mechanisms by implying IsShelleyBasedEra is part of the type-family dispatch, rather than a separate runtime constraint.

• Separate the two concerns explicitly:
  • Type families → compile-time; determine the types of arguments callers must supply
  • IsShelleyBasedEra era → runtime constraint providing the era singleton (ShelleyBasedEra era) for dispatching to the correct ledger operations
• Document caller requirements: polymorphic-era callers must include IsShelleyBasedEra era (or a stronger constraint implying it, e.g. IsBabbageBasedEra era) in their own type signatures

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[carbolymer]

this is redundant and does not bring a lot of new information