MINOR: activity: flush scheduler stats on "set profiling tasks on"

If a user enables profiling by hand, it makes sense to reset the stats
counters to provide fresh new measurements. Therefore it's worth using
this as the standard method to reset counters.
diff --git a/doc/management.txt b/doc/management.txt
index 99b3e59..e71849a 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1788,7 +1788,9 @@
 set profiling { tasks } { auto | on | off }
   Enables or disables CPU profiling for the indicated subsystem. This is
   equivalent to setting or clearing the "profiling" settings in the "global"
-  section of the configuration file. Please also see "show profiling".
+  section of the configuration file. Please also see "show profiling". Note
+  that manually setting the tasks profiling to "on" automatically resets the
+  scheduler statistics, thus allows to check activity over a given interval.
 
 set rate-limit connections global <value>
   Change the process-wide connection rate limit, which is set by the global