QProcess: Extend the setStandardOutputFile doc

It seems to be a hidden gem that one can stop redirecting the standard
output to a file without creating a new QProcess instance.

I have discovered this while browsing the private implementation of
QProcess trying to answer this question:

https://stackoverflow.com/a/72989131/2682142

I believe that this should be documented unless it is not meant to be
guaranteed behavior.

Change-Id: I180764d4cc56f6395d6d65942077e3ede63c71ea
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess.cpp

@@ -1378,6 +1378,9 @@ void QProcess::setStandardInputFile(const QString &fileName)
     Calling setStandardOutputFile() after the process has started has
     no effect.
 
+    If \a fileName is an empty string, it stops redirecting the standard
+    output. This is useful for restoring the standard output after redirection.
+
     \sa setStandardInputFile(), setStandardErrorFile(),
         setStandardOutputProcess()
 */