bootstrap: Describe build_steps::toolstate

Author: workingjubilee

src/bootstrap/src/core/build_steps/toolstate.rs

@@ -1,3 +1,7 @@
+//! Toolstate checks to keep tools building
+//!
+//! Reachable via `./x.py test` but mostly relevant for CI, since it isn't run locally by default.
+
 use crate::core::builder::{Builder, RunConfig, ShouldRun, Step};
 use crate::utils::helpers::t;
 use serde_derive::{Deserialize, Serialize};