doc(bootstrap): add top-level doc-comment to utils/tarball.rs

src/bootstrap/src/utils/tarball.rs

@@ -1,3 +1,10 @@
+//! Facilitates the management and generation of tarballs.
+//!
+//! Tarballs efficiently hold Rust compiler build artifacts and
+//! capture a snapshot of each boostrap stage.
+//! In uplifting, a tarball from Stage N captures essential components
+//! to assemble Stage N + 1 compiler.
+
 use std::{
     path::{Path, PathBuf},
     process::Command,