Added clarifications on watchdog assumptions

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

source/chapter1-about.rst

@@ -68,8 +68,9 @@ Assumptions
   the firmware as a single entity regardless of the components it comprises.
   Failing to update one of the components will lead to rollbacks of every affected component
 - Updating the firmware and the OS at the same time is prohibited.
-- A hardware watchdog must always be active at least in BL33.  It's advisable 
-  the watchdog is activated on earlier boot stages as well.
+- A hardware watchdog must always be active when Non-secure firmware is
+  executing.  It's advisable the watchdog is activated on earlier boot stages as well.
+  The watchdog must reset the board if a timeout occurs.
 
 .. [#UEFICapsuleUpdateNote] [UEFI]_ 2.8B § 23 - Firmware Update and Reporting