Clearer field descriptions for customers and Loyalty

49 response fields that had no description, or a vague one, now say what they hold and when they're empty.

• Retrieve customers: what counts as last seen, when points and VIP tiers expire, which units credit_balance_amount uses, and when yotpo_ugc is null. top_topics is now marked deprecated.
• Loyalty:
  • Campaigns: the multi-currency template fields, audience IDs and Birthday and Anniversary copy.
  • VIP tiers: entry rewards, how requirements combine and the period that thresholds count over.
  • Redemption options: type values, Recharge-only fields and how discount_rate_cents works.
  • Create a redemption: what token is for and when approved is false.
  • Identify a referrer: the referrer's discount code and each referral receipt.
• 423 errors now explain the cause: another request is changing the same customer. Wait a few seconds and retry.
• Delete customer data: all_stores: true anonymizes the customer in every store in your Yotpo organization.