Document clippy on nightly-rustc

src/bootstrap/builder.rs

@@ -482,6 +482,7 @@ impl<'a> Builder<'a> {
                 doc::RustByExample,
                 doc::RustcBook,
                 doc::CargoBook,
+                doc::Clippy,
                 doc::EmbeddedBook,
                 doc::EditionGuide,
             ),

src/bootstrap/doc.rs

@@ -755,6 +755,7 @@ tool_doc!(
     "src/tools/rustfmt",
     ["rustfmt-nightly", "rustfmt-config_proc_macro"],
 );
+tool_doc!(Clippy, "clippy", "src/tools/clippy", ["clippy_utils"]);
 
 #[derive(Ord, PartialOrd, Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct ErrorIndex {