📝 Add auth callback URL info in custom oauth config

apps/docs/self-hosting/configuration.mdx

@@ -140,6 +140,8 @@ The Authorization callback URL should be `$NEXTAUTH_URL/api/auth/callback/azure-
 
 For `*_PATH` parameters, you can use dot notation to access nested properties (i.e. `account.name`).
 
+The Authorization callback URL should be: `$NEXTAUTH_URL/api/auth/callback/custom-oauth`
+
 ## S3 Storage (Media uploads)
 
 Used for uploading images, videos, etc... It can be any S3 compatible object storage service (Minio, Digital Oceans Space, AWS S3...)