Document usage of os._exit() for exiting

In some cases, like when converting to html normal exit() makes
LibreOffice segfault, write to use os._exit() instead to
avoid that.

This is a known usage pattern, see lloconv:
https://github.com/ojwb/lloconv/commit/8f308cdb7c7e7f7230fee0172254d049d8bebc8d

Opened a bug here, still untriaged:
https://bugs.freedesktop.org/show_bug.cgi?id=82937

Fix #4