Update --enable-users documentation to specify the roles that support the permission

The documentation mentions that for --enable-users, you need "cluster.admin.security". It is actually much more complicated than that, so the docs should explain/summarise the below precise behaviour:

In addition, the specific users that get backed up depends on the roles of the user used to perform the backup (in addition to the actual Backup role being required):

• If the user has Full Admin, then all users will be included in the backup
• If the user has Local User Security Admin then local users will be included in the backup.
• If the user has External User Security Admin then external users will be included in the backup.
• If the user has Read-Only Admin then all users will be included in the backup

When performing a restore, the user will only be allowed to restore users if it has permission to create those same users. This means the following:

• For Full Admin, restoring users will always be permitted
• For Local User Security Admin, restoring users from a backup containing only local users is permitted
• For External User Security Admin, restoring users from a backup containing only external users is permitted
• For Read-Only Admin, restoring users is not permitted.

The middle two roles can be combined, which would give a user that can backup non-admin users and restore backups containing only non-admin users.

All of the above is assuming that the user has whatever other permissions are required to perform the rest of the backup/restore (without --enable-users).