How about grouping commands into sections?
If extension alias listed in ext-list is included in a section title, a user can easily know which commands are available.

Core Features:
  ext-list                    List all exts.
  ext-show                    Show information of a given resource
  net-create                  Create a network for a given tenant.
  net-delete                  Delete a given network.
  net-external-list           List external networks that belong to a given tenant
  net-list                    List networks that belong to a given tenant.
  net-show                    Show information of a given network.
  net-update                  Update network's information.
  port-create                 Create a port for a given tenant.
  port-delete                 Delete a given port.
  port-list                   List ports that belong to a given tenant.
  port-show                   Show information of a given port.
  port-update                 Update port's information.
  subnet-create               Create a subnet for a given tenant.
  subnet-delete               Delete a given subnet.
  subnet-list                 List networks that belong to a given tenant.
  subnet-show                 Show information of a given subnet.
  subnet-update               Update subnet's information.

Quota Extension:
  quota-delete                Delete defined quotas of a given tenant.
  quota-list                  List defined quotas of all tenants.
  quota-show                  Show quotas of a given tenant
  quota-update                Define tenant's quotas not to use defaults.

Layer-3 Extension:
  floatingip-associate        Create a mapping between a floating ip and a fixed ip.
  floatingip-create           Create a floating ip for a given tenant.
  floatingip-delete           Delete a given floating ip.
  floatingip-disassociate     Remove a mapping from a floating ip to a fixed ip.
  floatingip-list             List floating ips that belong to a given tenant.
  floatingip-show             Show information of a given floating ip.
  router-create               Create a router for a given tenant.
  router-delete               Delete a given router.
  router-gateway-clear        Remove an external network gateway from a router.
  router-gateway-set          Set the external network gateway for a router.
  router-interface-add        Add an internal network interface to a router.
  router-interface-delete     Remove an internal network interface from a router.
  router-list                 List routers that belong to a given tenant.
  router-port-list            List ports that belong to a given tenant, with specified router
  router-show                 Show information of a given router.
  router-update               Update router's information.

Security Gruop Extension:
  security-group-create       Create a security group.
  security-group-delete       Delete a given security group.
  security-group-list         List security groups that belong to a given tenant.
  security-group-rule-create  Create a security group rule.
  security-group-rule-delete  Delete a given security group rule.
  security-group-rule-list    List security group rules that belong to a given tenant.
  security-group-rule-show    Show information of a given security group rule.
  security-group-show         Show information of a given security group.

Quantum command itself:
  help                        print detailed help for another command