QFileSystemWatcher: add note about gedit and vim

Gedit and vim (possibly other 3rd parties editors) have a strange effect on how does QFileSystemWatcher work - the file path will be removed from the watcher list after a file modification with gedit/vim.

echoing data to the file will not. So it is somehow editor depended issue and basically out of scopes of Qt. However, you might want to warn users about this issue in QFileSystemWatcher documentation.