Rollup merge of #66655 - ollie27:rustdoc_extern-private_unstable, r=GuillaumeGomez

rustdoc: Mark `--extern-private` as unstable

It's not even stable in rustc so it shouldn't be stable in rustdoc.

r? @kinnison

src/test/rustdoc/issue-66159.rs

@@ -1,4 +1,5 @@
 // aux-build:issue-66159-1.rs
+// compile-flags:-Z unstable-options
 // extern-private:issue_66159_1
 
 // The issue was an ICE which meant that we never actually generated the docs