Allow adding a set of cfg's to hide from being implicitly doc(cfg)'d

By adding #![doc(cfg_hide(foobar))] to the crate attributes the cfg
 #[cfg(foobar)] (and _only_ that _exact_ cfg) will not be implicitly
treated as a doc(cfg) to render a message in the documentation.

compiler/rustc_span/src/symbol.rs

@@ -399,6 +399,7 @@ symbols! {
         cfg_attr_multi,
         cfg_doctest,
         cfg_eval,
+        cfg_hide,
         cfg_panic,
         cfg_sanitize,
         cfg_target_abi,
@@ -547,6 +548,7 @@ symbols! {
         doc,
         doc_alias,
         doc_cfg,
+        doc_cfg_hide,
         doc_keyword,
         doc_masked,
         doc_notable_trait,