Document one more requirement.

config/chroot_local-includes/usr/local/sbin/tails-debugging-info

@@ -31,8 +31,9 @@ debug_command grep spoof-mac: /var/log/messages
 # accessible to the user running this script (typicall the live
 # user). Therefore, when adding a new file, give as the first argument
 # 'root' only if the complete path to it (including the file itself)
-# is owned by root. If not, the following rules must be followed
-# strictly:
+# is owned by root and already exists before the system is connected to
+# the network (that is, before GDM's PostLogin script is run).
+# If not, the following rules must be followed strictly:
 #
 # * only one non-root user is involved in the ownership situation (the
 #   file, its dir and the parent dirs). From now on let's assume it is