Eliminate cfg(doc)

[dtolnay]

See dtolnay/ghost#20.

error[E0392]: parameter `T` is never used
   --> src/lib.rs:242:21
    |
242 |     pub struct iter<T>;
    |                     ^ unused parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `core::marker::PhantomData`
    = help: if you intended `T` to be a const parameter, use `const T: usize` instead