Documentation/trace: Add clarification how histogram onmatch works

The current trace documentation, the section describing histogram's "onmatch"
is not straightforward enough about how this action is applied. It is not
clear what criteria are used to "match" both events. A short note is added,
describing what exactly is compared in order to match the events.
Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Tom Zanussi <tom.zanussi@linux.intel.com>
[jc: fixed trivial conflict with docs-next]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/histogram.rst

@@ -1915,7 +1915,10 @@ The following commonly-used handler.action pairs are available:
 
     The 'matching.event' specification is simply the fully qualified
     event name of the event that matches the target event for the
-    onmatch() functionality, in the form 'system.event_name'.
+    onmatch() functionality, in the form 'system.event_name'. Histogram
+    keys of both events are compared to find if events match. In case
+    multiple histogram keys are used, they all must match in the specified
+    order.
 
     Finally, the number and type of variables/fields in the 'param
     list' must match the number and types of the fields in the
@@ -1978,9 +1981,9 @@ The following commonly-used handler.action pairs are available:
 	      /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
 
     Then, when the corresponding thread is actually scheduled onto the
-    CPU by a sched_switch event, calculate the latency and use that
-    along with another variable and an event field to generate a
-    wakeup_latency synthetic event::
+    CPU by a sched_switch event (saved_pid matches next_pid), calculate
+    the latency and use that along with another variable and an event field
+    to generate a wakeup_latency synthetic event::
 
       # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
               onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\