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_ast_passes/src/feature_gate.rs

@@ -279,6 +279,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
                 gate_doc!(
                     cfg => doc_cfg
+                    cfg_hide => doc_cfg_hide
                     masked => doc_masked
                     notable_trait => doc_notable_trait
                     keyword => doc_keyword