doc: fix lxc network load-balancer backend add command syntax

[minaelee]

This PR removes an unnecessary <listen_ports> option in the syntax for lxc network load-balancer backend add. This is correct in the man page but incorrect in the how-to guide this PR fixes. There is no listen_ports backend property. listen_ports is a port property that is used in lxc network load-balancer port add.

[minaelee]

@tomponline: I'm also concerned about this line that appears in the docs, right under the syntax for lxc network load-balancer backend add:

The target ports are optional. If not specified, the load balancer will use the listen ports for the backend for the backend target ports.

Since we've established that there are is no listen_port property of the load balancer backend, only of the load balancer port, the phrase listen ports for the backend doesn't seem applicable. Should this read ...the listen ports defined in the load balancer port properties... instead?

This line in the source seems to indicate that is the case. That the target ports, if not specified, fall back to the listen ports on the NetworkLoadBalancerPort.

If that's true, would I be correct to assume that whenever a backend is created, if no port options (such as listen_port) are configured explicitly, there is a default set of port options configured on the load balancer that includes a listen_port?

[tomponline]

@tomponline: I'm also concerned about this line that appears in the docs, right under the syntax for lxc network load-balancer backend add:

The target ports are optional. If not specified, the load balancer will use the listen ports for the backend for the backend target ports.

Since we've established that there are is no listen_port property of the load balancer backend, only of the load balancer port, the phrase listen ports for the backend doesn't seem applicable. Should this read ...the listen ports defined in the load balancer port properties... instead?

This line in the source seems to indicate that is the case. That the target ports, if not specified, fall back to the listen ports on the NetworkLoadBalancerPort.

If that's true, would I be correct to assume that whenever a backend is created, if no port options (such as listen_port) are configured explicitly, there is a default set of port options configured on the load balancer that includes a listen_port?

Yeah I agree.

Suggest something like

The target ports are optional. If not specified, the load balancer listener ports will be used for the backend target ports.

If that's true, would I be correct to assume that whenever a backend is created, if no port options (such as listen_port) are configured explicitly, there is a default set of port options configured on the load balancer that includes a listen_port?

No thats not correct, creating a load balancer simply reserves a listen IP (and no ports) then listen ports are added using:

lxc network load-balancer port add