Can it be added to the description? Otherwise, people new to the game are going to be very confused. There's already enough stuff to confuse them. Maybe also add something to the flavor text. "Cleric's channel the essence of the city to provide healing..."
Actually, maybe add the requirement to all descriptions You've got Provides now, then under that you could add Requires or Upgrades From. That would help people learn the game and also help them understand why they can build an improvement in one city and not another, or why they can't manually place an improvement (because it's an upgrade). One of the biggest challenges for new players to overcome is simply learning the game, and the existing documentation doesn't help too much.