rustdoc: add doc comment to DocVisitor

Author: lolbinarycat

src/librustdoc/visit.rs

@@ -1,5 +1,11 @@
 use crate::clean::*;
 
+/// allows a type to traverse the cleaned as of a crate.
+///
+/// note that like [`rustc_ast::visit::Visitor`], but
+/// unlike [`rustc_lint::EarlyLintPass`], if you override a
+/// `visit_*` method, you will need to manually recurse over
+/// its contents.
 pub(crate) trait DocVisitor: Sized {
     fn visit_item(&mut self, item: &Item) {
         self.visit_item_recur(item)
@@ -58,6 +64,7 @@ pub(crate) trait DocVisitor: Sized {
         m.items.iter().for_each(|i| self.visit_item(i))
     }
 
+    /// the main entrypoint of [`DocVisitor`].
     fn visit_crate(&mut self, c: &Crate) {
         self.visit_item(&c.module);