minor symfony#8146 Added a tip about the support of microseconds in Stopwatch (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes symfony#8146).

Discussion
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Added a tip about the support of microseconds in Stopwatch

This documents symfony/symfony#23223. See also https://symfony.com/blog/new-in-symfony-3-4-stopwatch-improvements.

Commits
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24718a6 Added a tip about the support of microseconds in Stopwatch

Author: xabbuh

components/stopwatch.rst

@@ -34,13 +34,24 @@ microtime by yourself. Instead, use the simple
     $event = $stopwatch->stop('eventName');
 
 The :class:`Symfony\\Component\\Stopwatch\\StopwatchEvent` object can be retrieved
-from the  :method:`Symfony\\Component\\Stopwatch\\Stopwatch::start`, 
-:method:`Symfony\\Component\\Stopwatch\\Stopwatch::stop`, 
-:method:`Symfony\\Component\\Stopwatch\\Stopwatch::lap` and 
-:method:`Symfony\\Component\\Stopwatch\\Stopwatch::getEvent` methods. 
+from the  :method:`Symfony\\Component\\Stopwatch\\Stopwatch::start`,
+:method:`Symfony\\Component\\Stopwatch\\Stopwatch::stop`,
+:method:`Symfony\\Component\\Stopwatch\\Stopwatch::lap` and
+:method:`Symfony\\Component\\Stopwatch\\Stopwatch::getEvent` methods.
 The latter should be used when you need to retrieve the duration of an event
 while it is still running.
 
+.. tip::
+
+    By default, the stopwatch truncates any sub-millisecond time measure to ``0``,
+    so you can't measure microseconds or nanoseconds. If you need more precision,
+    pass ``true`` to the ``Stopwatch`` class constructor to enable full precision::
+
+        $stopwatch = new Stopwatch(true);
+
+    .. versionadded:: 3.4
+        The full precision feature was introduced in Symfony 3.4.
+
 You can also provide a category name to an event::
 
     $stopwatch->start('eventName', 'categoryName');