remove error code from #[rustc_variance] and document its remains

compiler/rustc_error_codes/src/error_codes/E0208.md

@@ -1 +1,46 @@
 #### This error code is internal to the compiler and will not be emitted with normal Rust code.
+#### Note: this error code is no longer emitted by the compiler.
+
+This error code shows the variance of a type's generic parameters.
+
+Erroneous code example:
+
+```compile_fail
+// NOTE: this feature is perma-unstable and should *only* be used for
+//       testing purposes.
+#![feature(rustc_attrs)]
+
+#[rustc_variance]
+struct Foo<'a, T> { // error: deliberate error to display type's variance
+    t: &'a mut T,
+}
+```
+
+which produces the following error:
+
+```text
+error: [-, o]
+ --> <anon>:4:1
+  |
+4 | struct Foo<'a, T> {
+  | ^^^^^^^^^^^^^^^^^
+```
+
+*Note that while `#[rustc_variance]` still exists and is used within the*
+*compiler, it no longer is marked as `E0208` and instead has no error code.*
+
+This error is deliberately triggered with the `#[rustc_variance]` attribute
+(`#![feature(rustc_attrs)]` must be enabled) and helps to show you the variance
+of the type's generic parameters. You can read more about variance and
+subtyping in [this section of the Rustnomicon]. For a more in depth look at
+variance (including a more complete list of common variances) see
+[this section of the Reference]. For information on how variance is implemented
+in the compiler, see [this section of `rustc-dev-guide`].
+
+This error can be easily fixed by removing the `#[rustc_variance]` attribute,
+the compiler's suggestion to comment it out can be applied automatically with
+`rustfix`.
+
+[this section of the Rustnomicon]: https://doc.rust-lang.org/nomicon/subtyping.html
+[this section of the Reference]: https://doc.rust-lang.org/reference/subtyping.html#variance
+[this section of `rustc-dev-guide`]: https://rustc-dev-guide.rust-lang.org/variance.html