Remove misleading section about subprocesses

Perhaps I'm misunderstanding something, but I believe the following

The error codes returned by Gio.Subprocess.get_exit_status() are not the typical errors return by Gio functions. They can translated with the function [Gio.io_error_from_errno()][io_error_from_errno]

is just plain wrong. It suggests that get_exit_status returns errno codes, but in fact it returns the numeric exit code of the process which is something entirely different. I've never actually seen any program that returns actual errno codes as process exit codes.