Opening a file with WriteOnly truncates it

When opening a file with QIODevice::WriteOnly, it gets truncated.

The documentation for OpenModeFlag doesn't mention this and the presence of QIODevice::Truncate implies that WriteOnly alone does not truncate the file.

Attached is a file that reproduces the issue. The expected contents of test_file.txt after running the program are "testtest", but it ends up empty.

The docs need to be clearer.