OK, I think I understand this now, and the Help text is actually correct as it stands.
Each order, it turns out, has a commerce_line_items [note the plural] field attached to it, and it is this field, of unlimited cardinality, that stores all of the commerce_order/commerce_line_item pairs that are associated with this order. What this relationship does, then, is make the commerce_line_item reference field that is stored in the commerce_line_items field available to the view designer. Once this commerce_line_item reference field is available, all of the fields in the commerce_line_item itself become available.
The thing that was confusing is that the commerce_line_items field does not show up in a Site Builder's "Manage Fields" configuration screens, and I had not been able to find it in the code. Many thanks to argiepiano, who showed me where to find the definition of the commerce_line_items field [in the commerce_order module], and the place where data is saved into that field [in the commerce_cart module].
So that is the way that this all works ...
... I think.