Rollup merge of #90420 - GuillaumeGomez:rustdoc-internals-feature, r=camelid

Create rustdoc_internals feature gate

As suggested by ``@camelid`` [here](https://github.com/rust-lang/rust/pull/90398#issuecomment-955093851), since `doc_keyword` and `doc_primitive` aren't meant to be stabilized, we could put them behind a same feature flag.

This is pretty much what it would look like (needs to update the tests too).

The tracking issue is https://github.com/rust-lang/rust/issues/90418.

What do you think ``@rust-lang/rustdoc`` ?

src/test/rustdoc-gui/src/test_docs/lib.rs

@@ -2,7 +2,7 @@
 //! documentation generated so we can test each different features.
 
 #![crate_name = "test_docs"]
-#![feature(doc_keyword)]
+#![feature(rustdoc_internals)]
 #![feature(doc_cfg)]
 
 use std::convert::AsRef;