Explain 'management' network better

[jputrino]

from @pjbreaux:

In docs/guides/includes/os_ve_deploy_config-mgmt-network.rst:

> @@ -0,0 +1,74 @@
> +.. _os_ve_deploy_config-mgmt-network:
> +
> +1. Create a network to be used for the BIG-IP management interface.

maybe a quick blurb about what we mean when we say 'management'. As I understand it, it is used for control traffic, such as ssh access and VE configuration over REST.