Reuse logic for determining the channel in the rest of rustdoc

This doesn't update main.js because it's included as a fixed string.

src/librustdoc/core.rs

@@ -498,15 +498,18 @@ crate fn run_global_ctxt(
     let mut krate = tcx.sess.time("clean_crate", || clean::krate(&mut ctxt));
 
     if krate.module.doc_value().map(|d| d.is_empty()).unwrap_or(true) {
-        let help = "The following guide may be of use:\n\
-                https://doc.rust-lang.org/nightly/rustdoc/how-to-write-documentation.html";
+        let help = format!(
+            "The following guide may be of use:\n\
+            https://doc.rust-lang.org/{}/rustdoc/how-to-write-documentation.html",
+            crate::doc_rust_lang_org_channel(),
+        );
         tcx.struct_lint_node(
             crate::lint::MISSING_CRATE_LEVEL_DOCS,
             DocContext::as_local_hir_id(tcx, krate.module.def_id).unwrap(),
             |lint| {
                 let mut diag =
                     lint.build("no documentation found for this crate's top-level module");
-                diag.help(help);
+                diag.help(&help);
                 diag.emit();
             },
         );