Doc: Add documentation for added QNativeIpcKey overload

Document new implicit constructor introduced in the commit 08605f0d78

Pick-to: 6.6
Fixes: QTBUG-116870
Change-Id: I518f0a043d7738698ae568ce0b51db3f2c28574f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>

src/corelib/ipc/qtipccommon.cpp

@@ -345,6 +345,12 @@ QNativeIpcKey::Type QNativeIpcKey::defaultTypeForOs_internal() noexcept
 }
 #endif
 
+/*!
+    \fn QNativeIpcKey::QNativeIpcKey() noexcept
+
+    Constructs a QNativeIpcKey object of type \l DefaultTypeForOs with an empty key.
+*/
+
 /*!
     \fn QNativeIpcKey::QNativeIpcKey(Type type) noexcept
     \fn QNativeIpcKey::QNativeIpcKey(const QString &key, Type type)