When former late-night TV host Jay Leno petitioned a Los Angeles court to make him the conservator of his wife, Mavis, who has dementia, it focused attention on conservatorships, a sort of legal proceeding that’s an option when a person is incapacitated and unable to make important decisions by themselves. In a conservatorship in California — in many states, they’re called guardianships — a court gives someone, often a family member, the authority to control another person’s affairs and make...