doc: add a note to QFileSystemWatcher about files saved by re-creation

Fixes: QTBUG-46483
Fixes: QTBUG-53607
Change-Id: I434f177b35689d55aa1deea360bf7e6ae009ad68
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>

src/corelib/io/qfilesystemwatcher.cpp

@@ -451,6 +451,12 @@ QStringList QFileSystemWatcher::removePaths(const QStringList &paths)
     This signal is emitted when the file at the specified \a path is
     modified, renamed or removed from disk.
 
+    \note As a safety measure, many applications save an open file by
+    writing a new file and then deleting the old one. In your slot
+    function, you can check \c watcher.files().contains(path).
+    If it returns \c false, check whether the file still exists
+    and then call \c addPath() to continue watching it.
+
     \sa directoryChanged()
 */