Clarify use of prior_authorization and quantity_limit fields

[dportnoy]

Consider providing more explicit instructions for prior_authorization and quantity_limit fields. Currently they are currently labeled as optional. While these fields aren't applicable to all formulary entries, issuers should be instructed to specify them in cases where they apply. At the moment, it seems that many plans are leaving out these fields entirely, possibly because they're labeled as optional, without more explicit instruction. (See demand-driven-open-data/ddod-intake#52)