[Web Billing] Added limitation about upgrades (non virtual currency) (#1055)

* Added limitation

* Update docs/web/web-billing/overview.mdx

Co-authored-by: Ed Shelley <[email protected]>

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Co-authored-by: Ed Shelley <[email protected]>

Author: GuilhermeMota93

docs/web/web-billing/overview.mdx

@@ -60,6 +60,7 @@ Web Billing currently has a few limitations:
 - There is no support for RevenueCat's Paywall templates (coming soon).
 - There is no support for translated content in customer emails (these are currently only sent in English, regardless of the selected locale).
 - We do not collect and store the customer's name, shipping address or full billing address. Consequently, Web Billing cannot currently be used in India and in other countries that have this requirement.
+- There is no support for product downgrades (switching to lower-priced products). Customers need to cancel their existing subscription, and re-subscribe to the new product.
 
 ### Stripe Billing integration