logging: Clarify that QT_MESSAGE_PATTERN only applies to unstructured logging

Structured sinks such as systemd should capture each piece of information
individually, not bake it into the message via the message pattern.

Change-Id: I164c043683f123764a5445dc9faad049e25fd738
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/global/qlogging.cpp

@@ -1886,6 +1886,10 @@ void qErrnoWarning(int code, const char *msg, ...)
     environment variable; if both \l qSetMessagePattern() is called and QT_MESSAGE_PATTERN is
     set, the environment variable takes precedence.
 
+    \note The message pattern only applies to unstructured logging, such as the default
+    \c stderr output. Structured logging such as systemd will record the message as is,
+    along with as much structured information as can be captured.
+
     Custom message handlers can use qFormatLogMessage() to take \a pattern into account.
 
     \sa qInstallMessageHandler(), {Debugging Techniques}, {QLoggingCategory}