minor #12639 Add isMasterRequest method (makmaoui)

This PR was submitted for the 4.3 branch but it was merged into the 3.4 branch instead (closes #12639).

Discussion
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Add isMasterRequest method

The isMasterRequest method is available on KernelEvent but not listed in documentation

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Commits
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0cf2b81 Add isMasterRequest method

Author: xabbuh

reference/events.rst

@@ -23,6 +23,9 @@ following information:
 :method:`Symfony\\Component\\HttpKernel\\Event\\KernelEvent::getRequest`
     Returns the current ``Request`` being handled.
 
+:method:`Symfony\\Component\\HttpKernel\\Event\\KernelEvent::isMasterRequest`
+    Checks if this is a master request.
+
 .. _kernel-core-request:
 
 ``kernel.request``