doc/performance: clean up information about txqueuelen
doc/performance: clean up information about txqueuelen

Clean up the page about how to increase the network bandwidth.

Signed-off-by: Ruth Fuchss <90c286bb8b7c8291b9f5a2a1d38dc233728381fe@canonical.com>

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