Added brief comments.
diff --git a/PCbuild/pyclean.bat b/PCbuild/pyclean.bat
index cad60b6..d28262e 100644
--- a/PCbuild/pyclean.bat
+++ b/PCbuild/pyclean.bat
@@ -1,3 +1,3 @@
+@rem Used by the buildbot "clean" step.
 call "%VS71COMNTOOLS%vsvars32.bat"
 devenv.com /clean Debug PCbuild\pcbuild.sln
-