Remove the E0536 error code

Author: JonathanBrouwer

compiler/rustc_error_codes/src/error_codes/E0536.md

@@ -1,10 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The `not` cfg-predicate was malformed.
 
-Erroneous code example (using `cargo doc`):
+Erroneous code example:
 
-```ignore, E0536 (only triggers on cargo doc)
-#![feature(doc_cfg)]
-#[doc(cfg(not()))]
+```compile_fail
+#[cfg(not())]
 pub fn main() {
 
 }
@@ -13,8 +14,7 @@ pub fn main() {
 The `not` predicate expects one cfg-pattern. Example:
 
 ```
-#![feature(doc_cfg)]
-#[doc(cfg(not(target_os = "linux")))] // ok!
+#[cfg(not(target_os = "linux"))] // ok!
 pub fn main() {
 
 }