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/libcore/lib.rs

@@ -98,6 +98,7 @@
 #![feature(is_sorted)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
+#![cfg_attr(not(bootstrap), feature(negative_impls))]
 #![feature(never_type)]
 #![feature(nll)]
 #![feature(exhaustive_patterns)]