[doc] Warn users about data races regarding qt_ntfs_permission_lookup

qt_ntfs_permission_lookup is a non-atomic global variable that is prone
to data races. Make a remark about this in the documentation.

Task-number: QTBUG-105804
Pick-to: 6.5 6.4 6.2 5.15
Change-Id: If7c64f3ab7d2c3b1487fe56204a4e66c420b0604
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/io/qfiledevice.cpp

@@ -109,6 +109,11 @@ void QFileDevicePrivate::setError(QFileDevice::FileError err, int errNum)
     decrementing \c qt_ntfs_permission_lookup by 1.
 
     \snippet ntfsp.cpp 1
+
+    \note Since this is a non-atomic global variable, it is only safe
+    to increment or decrement \c qt_ntfs_permission_lookup before any
+    threads other than the main thread have started or after every thread
+    other than the main thread has ended.
 */
 
 //************* QFileDevice

src/corelib/io/qfileinfo.cpp

@@ -273,6 +273,11 @@ QDateTime &QFileInfoPrivate::getFileTime(QAbstractFileEngine::FileTime request)
 
     \snippet ntfsp.cpp 1
 
+    \note Since this is a non-atomic global variable, it is only safe
+    to increment or decrement \c qt_ntfs_permission_lookup before any
+    threads other than the main thread have started or after every thread
+    other than the main thread has ended.
+
     \section1 Performance Considerations
 
     Some of QFileInfo's functions query the file system, but for