pdf documentation

[yantux]

pdf documentation contain info aboun installing VirtualBox on Linux. i.e. 'usermod -a -G virtualbox user', but in some linux distribution usermod utility did n ot support -a/--append option. Because, if linux user is not has good knowledge, then he delete -a option. After that user lose own groups and get obnly virtualbox group. May be add info in pdf and message box, that admin can use not onlu usermod, but and gpasswd? Because it utility more easy than usermod.

Thank you.

Best regards, Yan.