compiler/rustc_error_codes/src/error_codes/E0705.md

A `#![feature]` attribute was declared for a feature that is stable in the
current edition, but not in all editions.

Erroneous code example:

```rust2018,compile_fail,E0705
#![feature(rust_2018_preview)]
#![feature(test_2018_feature)] // error: the feature
                               // `test_2018_feature` is
                               // included in the Rust 2018 edition
```