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`` ?

compiler/rustc_span/src/symbol.rs

@@ -1155,6 +1155,7 @@ symbols! {
         rustc_unsafe_specialization_marker,
         rustc_variance,
         rustdoc,
+        rustdoc_internals,
         rustfmt,
         rvalue_static_promotion,
         s,