Webhook application ID, rate limits and response fields corrected

We tested the API reference against live calls and fixed what didn't match:

Webhooks: in /core/v3/applications/{application_id}/..., use your app's client ID (the 32-character Application ID you got at registration) as application_id. The numeric app ID returns 403 Forbidden.
Rate limits: Guidelines and Conventions now lists the real limits per endpoint group, counted per app per store, and the RateLimit-* headers on every response. Over the limit, you get 429 with a Retry-After header.
Retrieve Loyalty Customer Details: wrap the body in a customers object. A customer that isn't in the loyalty program returns 404.
New response fields:
- external_variant_id on order line items.
- Image sizes on All Products.
- is_incentivized, incentive_type and user_type on reviews.
- A full response schema for Product Data.
Errors: app-level webhook endpoints no longer list a source field, which they don't return. Promoted Products documents the 401 returned when the store doesn't have the feature, and Retrieve Existing Comment the 404 for a review without a comment.

Endpoints affected: all webhook target, filter and subscription endpoints, Retrieve Orders, Retrieve order by ID, All Products, Product Data, All Reviews, Review by ID, Promoted Products, Retrieve Existing Comment, Retrieve Loyalty Customer Details, Guidelines and Conventions, Errors.