bootstrap: Describe build_steps::run

src/bootstrap/src/core/build_steps/run.rs

@@ -1,3 +1,8 @@
+//! Build-and-run steps for in-repo tools
+//!
+//! A bit of a hodge-podge as e.g. if a tool's a test fixture it should be in `build_steps::test`.
+//! If it can be reached from `./x.py run` it can go here.
+
 use std::path::PathBuf;
 use std::process::Command;