test.pl: Add comment that you should clear watchdog

As it says, failure to cancel a timer can lead to failing (or worse:
flapping) tests

t/test.pl

@@ -1768,6 +1768,21 @@ sub warning_like {
 # variable in your profile for slow boxes, or use it to override the timeout
 # temporarily for debugging.
 #
+# It is good practice to CLEAR EVERY WATCHDOG timer.  Otherwise the timer
+# applies to the entire rest of the file.  Even if that works now, new tests
+# tend to get added to the end of the file, and people may not notice that
+# they are being timed.  Those tests may all complete before the timer kills
+# them, but then more new tests get added, even further away from the timer
+# setting code, with less likelihood of noticing that.  Those tests may also
+# generally work, but flap on heavily loaded smokers, leading to debugging
+# effort that wouldn't have had to be expended if the timer had been cancelled
+# in the first place
+#
+# This will figure out a suitable method to implement the timer, but you can
+# force it to use an alarm by setting the optional second parameter to
+# 'alarm', or to use a separate process (if available on this platform) by
+# setting that parameter to 'process'.
+#
 # NOTE:  If the test file uses 'threads', then call the watchdog() function
 #        _AFTER_ the 'threads' module is loaded.
 { # Closure