Update comments to reflect current USE_SCREEN usage

In a couple of places the tracking of USE_SCREEN has drifted from the
comments.

Correct that.

Change-Id: I63bdd5ca4de49bf653f5bc8f8e0e5efe67ef605c
diff --git a/stackrc b/stackrc
index 76a5756..4f45939 100644
--- a/stackrc
+++ b/stackrc
@@ -101,7 +101,7 @@
 # ctrl-c, up-arrow, enter to restart the service. Starting services
 # this way is slightly unreliable, and a bit slower, so this can
 # be disabled for automated testing by setting this value to False.
-USE_SCREEN=True
+USE_SCREEN=$(trueorfalse True USE_SCREEN)
 
 # When using screen, should we keep a log file on disk?  You might
 # want this False if you have a long-running setup where verbose logs