rust: init: remove doctest's `Error::from_errno` workaround

//! #     pub unsafe fn destroy_foo(_ptr: *mut foo) {}

//! #     pub unsafe fn enable_foo(_ptr: *mut foo, _flags: u32) -> i32 { 0 }

//! # }

//! # // `Error::from_errno` is `pub(crate)` in the `kernel` crate, thus provide a workaround.

//! # trait FromErrno {

//! #     fn from_errno(errno: core::ffi::c_int) -> Error {

//! #         // Dummy error that can be constructed outside the `kernel` crate.

//! #         Error::from(core::fmt::Error)

//! #     }

//! # }

//! # impl FromErrno for Error {}

//! /// # Invariants

//! ///

//! /// `foo` is always initialized