Rollup merge of #81492 - camelid:rustdoc-internal-mod-vis, r=GuillaumeGomez

rustdoc: Note why `rustdoc::html::markdown` is public

Almost all of the modules are crate-private, except for
`rustdoc::json::types`, which I believe is intended to be for public
use; and `rustdoc::html::markdown`, which is used externally by the
error-index generator and so has to be public.

r? ``@GuillaumeGomez``

src/librustdoc/lib.rs

@@ -83,7 +83,8 @@ mod doctree;
 mod error;
 mod doctest;
 mod fold;
-crate mod formats;
+mod formats;
+// used by the error-index generator, so it needs to be public
 pub mod html;
 mod json;
 mod markdown;