Gentle update to UserDataDir policy documentation.

As of M78, UserDataDir may point to the root of a volume.

BUG=619133
R=zmin@chromium.org

Change-Id: Ic9c004c388656abacb3cebf4c8faae4b625bb936
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148734
Auto-Submit: Greg Thompson <grt@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758828}

components/policy/resources/policy_templates.json

@@ -3226,7 +3226,7 @@
       'tags': ['local-data-access'],
       'desc': '''Configures the directory that <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use for storing user data.
 
-      If you set this policy, <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use the provided directory regardless whether the user has specified the '--user-data-dir' flag or not. To avoid data loss or other unexpected errors this policy should not be set to a volume's root directory or to a directory used for other purposes, because <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> manages its contents.
+      If you set this policy, <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use the provided directory regardless whether the user has specified the '--user-data-dir' flag or not. To avoid data loss or other unexpected errors this policy should not be set to a directory used for other purposes, because <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> manages its contents.
 
       See https://www.chromium.org/administrators/policy-list-3/user-data-directory-variables for a list of variables that can be used.