Added clarification about the userId and made it nullable

BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/UserAccount.kt

@@ -19,7 +19,17 @@ open class UserAccount(
     val bic: String,
 
     val customerName: String,
-    val userId: String = loginName,
+    /**
+     * The customer is the person or organisation that owns the account at the bank.
+     *
+     * The user is the person that access a customer's account via e.g. FinTS. The user may but not necessarily is
+     * identical with the customer. E.g. an employee from the bookkeeping departement or a tax consultant may only
+     * access an account for someone else.
+     *
+     * So in most cases the userId is identical with the customerId = loginName in our speech, but there are rare cases
+     * where the userId differs from customerId.
+     */
+    val userId: String? = null,
 
     open val accounts: List<BankAccount> = emptyList(),