feature #19912 [Stopwatch] Add ROOT constant to make it easier to reference (alamirault)

This PR was merged into the 7.2 branch.

Discussion
----------

[Stopwatch] Add `ROOT` constant to make it easier to reference

Fix #19907

Using constant is the best practice IMO

Commits
-------

01ca38d [Stopwatch] Add ROOT constant to make it easier to reference

Author: OskarStark

performance.rst

@@ -389,10 +389,16 @@ All events that don't belong to any named section are added to the special secti
 called ``__root__``. This way you can get all stopwatch events, even if you don't
 know their names, as follows::
 
-    foreach($this->stopwatch->getSectionEvents('__root__') as $event) {
+    use Symfony\Component\Stopwatch\Stopwatch;
+
+    foreach($this->stopwatch->getSectionEvents(Stopwatch::ROOT) as $event) {
         echo (string) $event;
     }
 
+.. versionadded:: 7.2
+
+    The ``Stopwatch::ROOT`` constant as a shortcut for ``__root__`` was introduced in Symfony 7.2.
+
 Learn more
 ----------