Documenting the __root__ section

Finally closes the (already closed) #13987

The info is taken from @stof at symfony/symfony#37630 (comment)

Author: ThomasLandauer

performance.rst

@@ -327,6 +327,15 @@ Sections are a way to split the profile timeline into groups. Example::
     $this->stopwatch->start('processing-file');
     $this->stopwatch->stopSection('parsing');
 
+All events that don't belong to any named section are added to the special section
+``__root__``. This way you can get all stopwatch events, even if you don't know
+their names::
+
+    foreach($this->stopwatch->getSectionEvents('__root__') as $event) {
+        echo (string) $event;
+    }
+
+
 Learn more
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