rustdoc: Add FIXME test for doc_cfg interaction with check_cfg

Author: clubby789

Diff for: tests/rustdoc-ui/doc-cfg-check-cfg.rs

@@ -0,0 +1,16 @@
+// Ensure that `doc(cfg())` respects `check-cfg`
+// Currently not properly working
+#![feature(doc_cfg)]
+#![deny(unexpected_cfgs)]
+
+//@revisions: no_check cfg_empty cfg_foo
+//@[cfg_empty] compile-flags: --check-cfg cfg()
+//@[cfg_foo] compile-flags: --check-cfg cfg(foo)
+
+//@[no_check] check-pass
+//@[cfg_empty] check-pass
+//@[cfg_empty] known-bug: #138358
+//@[cfg_foo] check-pass
+
+#[doc(cfg(foo))]
+pub fn foo() {}