Merge branch '5.4' into 6.0

* 5.4:
  Update section setting headers globally
  Documenting the __root__ section

Author: javiereguiluz

mailer.rst

@@ -542,6 +542,12 @@ and headers.
             $mailer->header('X-Custom-Header')->value('foobar');
         };
 
+.. caution::
+
+    Some third-party providers don't support the usage of keywords like ``from``
+    in the ``headers``. Check out your provider's documentation before setting
+    any global header.
+
 Handling Sending Failures
 -------------------------
 
@@ -1414,3 +1420,4 @@ you can use the built in assertions::
 .. _`PEM encoded`: https://en.wikipedia.org/wiki/Privacy-Enhanced_Mail
 .. _`default_socket_timeout`: https://www.php.net/manual/en/filesystem.configuration.php#ini.default-socket-timeout
 .. _`RFC 3986`: https://www.ietf.org/rfc/rfc3986.txt
+.. _`Mailjet API V3`: https://dev.mailjet.com/email/guides/send-api-v31/#add-email-headers

performance.rst

@@ -334,6 +334,14 @@ 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
+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) {
+        echo (string) $event;
+    }
+
 Learn more
 ----------