Add section on bulk assignment

What does this MR do and why?

Updates docs with information on new bulk assignment functionality for Duo Pro within GitLab.com. See https://gitlab.com/gitlab-org/gitlab/-/issues/455443 for more information.

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doc/subscriptions/subscription-add-ons.md

@@ -90,7 +90,7 @@ You can use bulk assignment to assign seats to multiple users at one time.
 1. Using the checkboxes to the left of each user, select the users for assignment. To select all, select the checkbox at the top of the table.
 1. Select **Assign seat** and then **Assign seats** again to confirm.
 
-To assign seats in bulk without using the UI, you can use [this GraphQL API endpoint](../api/graphql/reference/index.md#mutationuseraddonassignmentcreate).
+To assign seats in bulk without using the UI, you can use [this GraphQL API endpoint](../api/graphql/reference/index.md#mutationuseraddonassignmentbulkcreate).
 
 ### For self-managed