(billing): clarify Stripe account linking requirements (#3292)

Co-authored-by: Michael Novotny <manovotny@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

docs/guides/billing/overview.mdx

@@ -31,7 +31,11 @@ Clerk Billing allows your customers to purchase recurring Subscriptions to your
 
 ### Can I use an existing Stripe account with Clerk Billing?
 
-Yes, you can. However, it must not already be linked to another platform.
+Yes, you can use an existing Stripe account, as long as it isn't controlled by another platform.
+
+Disconnect accounts created under a platform's Stripe Connect setup from that platform before linking them to Clerk.
+
+In general, if you created your Stripe account yourself via Stripe, it's independent; if it was created through another service, it may be platform-controlled.
 
 ### Can I see Subscriptions in my Stripe account?