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/test/ui/issues/issue-17718-static-sync.rs

@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 use std::marker::Sync;
 

src/test/ui/issues/issue-23080-2.rs

@@ -1,6 +1,7 @@
 //~ ERROR
 
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 unsafe auto trait Trait {
     type Output; //~ ERROR E0380

src/test/ui/issues/issue-23080-2.stderr

@@ -1,5 +1,5 @@
 error[E0380]: auto traits cannot have methods or associated items
-  --> $DIR/issue-23080-2.rs:6:10
+  --> $DIR/issue-23080-2.rs:7:10
    |
 LL | unsafe auto trait Trait {
    |                   ----- auto trait cannot have items

src/test/ui/issues/issue-23080.rs

@@ -1,4 +1,5 @@
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 unsafe auto trait Trait {
     fn method(&self) { //~ ERROR E0380

src/test/ui/issues/issue-23080.stderr

@@ -1,5 +1,5 @@
 error[E0380]: auto traits cannot have methods or associated items
-  --> $DIR/issue-23080.rs:4:8
+  --> $DIR/issue-23080.rs:5:8
    |
 LL | unsafe auto trait Trait {
    |                   ----- auto trait cannot have items

src/test/ui/issues/issue-29516.rs

@@ -1,5 +1,6 @@
 // check-pass
 #![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 
 auto trait NotSame {}
 

src/test/ui/issues/issue-33140-hack-boundaries.rs

@@ -1,4 +1,4 @@
-#![feature(optin_builtin_traits)]
+#![feature(negative_impls)]
 #![allow(order_dependent_trait_objects)]
 
 // Check that the issue #33140 hack does not allow unintended things.