To match when the attribute on the bind DN entry corresponds directly to the attribute on the target entry, replace attr with the attribute type, and value with the attribute value.
When trying to understand this I stepped through the code. It does an internal search of the bind DN entry, requesting all user attributes. It might be worth clarifying that, as attempts to use an operational attribute will never work.
The "userattr" term is suggestive, but more clarity is always good.