lxc/config_trust: clarify help for join token creation
lxc/config_trust: clarify help for join token creation

Closes #10379.

Signed-off-by: Simon Deziel <eca3f1a14b730e70ce4898287e4fc2f9966fa3a6@canonical.com>

stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd