[user_accounts] Outdated TestPlan.md + potential permission issue

[jeffersoncasimir]

The TestPlan.md says the following:

15. Verify that if the editor does not have permission 'Across all sites add and edit users' then the site drop-down list is populated with the editor's associated sites, otherwise all sites are displayed.

Site is now a multi-select and this should be reflected in the documentation.

Note: The above quoted item is only true for the Edit User page. When at the index, the FilterableDataTable's Site filter contains all sites even if the user only has permissions for a single site and the User Accounts: View/Create/Edit User Accounts - Own Sites permission. This does not appear the be the case for other modules' FilterableDataTable.