Fix unclear message when deploying SELinux changes

Changing the security attributes of a file requires a build_image. Make
that clear in the message.

BUG=None
TEST=cros deploy chromeos-base/selinux-policy, saw new message

Change-Id: I4c4162b15802cc5cad3d8d16d2747665b6b004ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2355080
Tested-by: Ian Barkley-Yeung <iby@chromium.org>
Auto-Submit: Ian Barkley-Yeung <iby@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Commit-Queue: Dhanya Ganesh <dhanyaganesh@chromium.org>
diff --git a/cli/deploy.py b/cli/deploy.py
index 1633d58..5a71657 100644
--- a/cli/deploy.py
+++ b/cli/deploy.py
@@ -1209,9 +1209,9 @@
         if sum(x.count('selinux-policy') for x in pkgs):
           logging.warning(
               'Deploying SELinux policy will not take effect until reboot. '
-              'SELinux policy is loaded by init. Also, security contexts '
-              '(labels) in files will require manual relabeling by the user '
-              'if your policy modifies the file contexts.')
+              'SELinux policy is loaded by init. Also, changing the security '
+              'contexts (labels) of a file will require building a new image '
+              'and flashing the image onto the device.')
 
       logging.warning('Please restart any updated services on the device, '
                       'or just reboot it.')