Document why we pass truffleruby.mspec explicitly

Author: eregon

tool/jt.rb

@@ -703,7 +703,9 @@ def git_clone(url, path)
   end
 
   def run_mspec(env_vars, command = 'run', *args)
-    mspec_args = ['spec/mspec/bin/mspec', command, '--config', ENV['TRUFFLERUBY_MSPEC_CONFIG'] || 'spec/truffleruby.mspec']
+    # Pass spec/truffleruby.mspec explicitly because we also want to use it when
+    # running specs on CRuby so that it finds that specs are under spec/ruby.
+    mspec_args = ['spec/mspec/bin/mspec', command, '--config', 'spec/truffleruby.mspec']
 
     Dir.chdir(TRUFFLERUBY_DIR) do
       # always enable assertions with --ea to catch issues earlier