docs(open-payments): specify port for auth choice route

packages/documentation/src/content/docs/concepts/open-payments/grant-interaction.mdx

@@ -15,6 +15,7 @@ If the AS deems interaction necessary to issue a grant, there are five main endp
 - `GET /interact/:id/:nonce` (made by the client to the AS, establishes an interaction session, redirects browser session to IDP consent screen)
 - `GET /grant/:id/:nonce` (made by the IDP to the AS, secured with `x-idp-secret` header, returns grant info for the consent screen to enumerate )
 - `POST /grant/:id/:nonce/(accept OR reject)` (made by the IDP to the AS, secured with `x-idp-secret` header, accepts or rejects the grant based on the user's input on the consent screen. **IDP then redirects to `GET /interact/:id/:nonce/finish`**)
+  - **This is served on AUTH_CHOICE_PORT and its default value is 3009**
 - `GET /interact/:id/:nonce/finish` (ends the interaction established by `GET /interact/:id/:nonce`, redirects browser session to client callback. Contains a query param that either indicates a failure, or on success, a `hash` parameter that the client can use to verify the successful interaction, and the `interact_ref` that identifies the interaction on the AS.)
   - Examples include: - `?result=grant_rejected` (if interaction was rejected) - `?result=grant_invalid` (if grant is not in a state where it may be accepted or rejected, e.g. already approved) - `?hash=p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R\HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A
 &interact_ref=4IFWWIKYBC2PQ6U56NL1` (if interaction was accepted) - `hash` is a `sha-256` hash of values provided by the client in the body of the [grant initialization request](https://docs.openpayments.guide/reference/post-request) (`interact.finish.nonce`), values returned in the AS response for that request (`interact.finish`), the `interact_ref` provided alongside the `hash`, and the uri of the grant initialization request (`https://auth-server.com/`).