Document os_find_datadir() returned data must be freed.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael Rolnik <mrolnik@gmail.com>
Tested-by: Michael Rolnik <mrolnik@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200714164257.23330-2-f4bug@amsat.org>