add simple top-level doc-comment for build_steps/clippy

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/build_steps/clippy.rs

@@ -1,3 +1,5 @@
+//! Implementation of running clippy on the compiler, standard library and various tools.
+
 use std::path::Path;
 
 use crate::builder::Builder;