documentation: behavior of lfs_stat

[pabigot]

By experimentation lfs_file_stat() will not return the correct file size if the file is open and has been written to: a newly created and written file to which several block size values have been written will have size zero. The correct size is available after the file is closed.

I don't have a problem with this behavior, but it would be nice if the interaction of open files with lfs_file_stat were noted in the function documentation.

[pabigot]

An lfs_file_sync() operation can be used to get the correct size information of an open file.