minor #13311 Added documentation for __toString feature in StopWatch event (zf2timo)

This PR was squashed before being merged into the 3.4 branch (closes #13311).

Discussion
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Added documentation for __toString feature in StopWatch event

I noticed there is no documentation about using the `__toString` method of the StopWatch Event. So here is it.

Commits
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1d85ccb Added documentation for __toString feature in StopWatch event

Author: javiereguiluz

components/stopwatch.rst

@@ -120,3 +120,16 @@ method and specifying the id of the section to be reopened::
     $stopwatch->openSection('routing');
     $stopwatch->start('building_config_tree');
     $stopwatch->stopSection('routing');
+
+
+Printing a Summary
+------------------
+
+The event can easily be printed to display a summary::
+
+    $stopwatch = new Stopwatch();
+    $stopwatch->start();
+    // ... do some work here
+    $output->writeln($stopwatch->stop()); // prints something like default: 14.00 MiB - 2056 ms
+
+This gives you a brief summary of the used memory an elapsed time.