introduce negative_impls feature gate and document

They used to be covered by `optin_builtin_traits` but negative impls
are now applicable to all traits, not just auto traits.

This also adds docs in the unstable book for the current state of auto traits.

src/librustc_errors/lib.rs

@@ -802,13 +802,13 @@ impl HandlerInner {
                     ));
                     self.failure(&format!(
                         "For more information about an error, try \
-                                           `rustc --explain {}`.",
+                         `rustc --explain {}`.",
                         &error_codes[0]
                     ));
                 } else {
                     self.failure(&format!(
                         "For more information about this error, try \
-                                           `rustc --explain {}`.",
+                         `rustc --explain {}`.",
                         &error_codes[0]
                     ));
                 }