Clarify comment about why bootstrap tests need src/doc/book

Author: ehuss

src/bootstrap/src/core/builder/tests.rs

@@ -13,9 +13,12 @@ fn configure_with_args(cmd: &[String], host: &[&str], target: &[&str]) -> Config
     config.save_toolstates = None;
     config.dry_run = DryRun::SelfCheck;
 
-    // Ignore most submodules, since we don't need them for a dry run.
-    // But make sure to check out the `doc` and `rust-analyzer` submodules, since some steps need them
-    // just to know which commands to run.
+    // Ignore most submodules, since we don't need them for a dry run, and the
+    // tests run much faster without them.
+    //
+    // The src/doc/book submodule is needed because TheBook step tries to
+    // access files even during a dry-run (may want to consider just skipping
+    // that in a dry run).
     let submodule_build = Build::new(Config {
         // don't include LLVM, so CI doesn't require ninja/cmake to be installed
         rust_codegen_backends: vec![],