Bug 1820237 - Mention the start-shell.bat script in code block r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D171600

docs/setup/windows_build.rst

@@ -74,6 +74,7 @@ the interactive setup process.
 
 .. code-block:: shell
 
+    # Using the C:\mozilla-build\start-shell.bat shell from step 1.2:
     cd c:/
     mkdir mozilla-source
     cd mozilla-source