Clarify addition of outside collaborators to cost centers (#60039)

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: ddivad195

content/billing/concepts/cost-centers.md

@@ -31,4 +31,4 @@ For more details, see [AUTOTITLE](/billing/reference/cost-center-allocation).
 * The maximum number of resources per cost center is 25,000.
 * A maximum of 50 resources can be added to or removed from a cost center at a time.
 * Azure subscriptions can only be added to or removed from cost centers through the UI.
-* Outside collaborators can only be added to cost centers via the cost center API. For more information, see [AUTOTITLE](/billing/tutorials/control-costs-at-scale#add-resources-to-the-cost-center).
+* Outside collaborators or unaffiliated users can only be added to cost centers via the cost center API. For more information, see [AUTOTITLE](/billing/tutorials/control-costs-at-scale#add-resources-to-the-cost-center).