The attribute can be applied to a newtype-like structs that contains a single field. It indicates that the newtype should be represented exactly like that field’s type, i.e., the newtype should be ignored for ABI purpopses: not only is it laid out the same in memory, it is also passed identically in function calls.
I find the older documentation clearer in relation to newtype because it clearly says: If you want the newtype exactly for type safety the repr transparent is what you what because memory layout and c-api calls are the them.
The current documentation means the same but there is no relation to the newtype pattern.
In fact the word newtype does is only once found in the current documentation. Does the newtype pattern have a new name?