update lxc-attach manpage
update lxc-attach manpage

- explain rationale behind allocation of pty
- briefly explain how a pty is allocated
- add a short note that describes the changed behavior for lxc-attach when the
  user is not placed in a writeable cgroup at login

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@mailbox.org>

GreatFruitOmsk/lxc,hallyn/lxc,aeris/lxc,ss1h2a3tw/lxc,caglar10ur/lxc,caglar10ur/lxc,aeris/lxc,terceiro/lxc,cjwatson/lxc,aeris/lxc,LynxChaus/lxc,thmo/lxc,thmo/lxc,cjwatson/lxc,hallyn/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,hallyn/lxc,hfuCN/lxc,tych0/lxc,GreatFruitOmsk/lxc,cjwatson/lxc,federicobriata/lxc,QingweiPeterLan/cs188-lxc,federicobriata/lxc,ysbnim/lxc,akshaykarle/lxc,caglar10ur/lxc,terceiro/lxc,hfuCN/lxc,hfuCN/lxc,tych0/lxc,tych0/lxc,federicobriata/lxc,thmo/lxc,akshaykarle/lxc,LynxChaus/lxc,terceiro/lxc,QingweiPeterLan/cs188-lxc,ysbnim/lxc,akshaykarle/lxc,QingweiPeterLan/cs188-lxc,ss1h2a3tw/lxc,ysbnim/lxc,GreatFruitOmsk/lxc