Doc: Add network resource descriptions

Add descriptions to the network resource command documentation.

Change-Id: I547ffb48f8950311a5ee65d6b535846f2aca0efc

doc/source/command-objects/availability-zone.rst

@@ -2,6 +2,9 @@
 availability zone
 =================
 
+An **availability zone** is a logical partition of cloud block storage,
+compute and network services.
+
 Block Storage v2, Compute v2, Network v2
 
 availability zone list

doc/source/command-objects/network.rst

@@ -2,6 +2,13 @@
 network
 =======
 
+A **network** is an isolated Layer 2 networking segment. There are two types
+of networks, project and provider networks. Project networks are fully isolated
+and are not shared with other projects. Provider networks map to existing
+physical networks in the data center and provide external network access for
+servers and other resources. Only an OpenStack administrator can create
+provider networks. Networks can be connected via routers.
+
 Compute v2, Network v2
 
 network create

doc/source/command-objects/port.rst

@@ -2,6 +2,10 @@
 port
 ====
 
+A **port** is a connection point for attaching a single device, such as the
+NIC of a server, to a network. The port also describes the associated network
+configuration, such as the MAC and IP addresses to be used on that port.
+
 Network v2
 
 port create

doc/source/command-objects/router.rst

@@ -2,6 +2,10 @@
 router
 ======
 
+A **router** is a logical component that forwards data packets between
+networks. It also provides Layer 3 and NAT forwarding to provide external
+network access for servers on project networks.
+
 Network v2
 
 router add port

doc/source/command-objects/security-group-rule.rst

@@ -2,6 +2,9 @@
 security group rule
 ===================
 
+A **security group rule** specifies the network access rules for servers
+and other resources on the network.
+
 Compute v2, Network v2
 
 security group rule create

doc/source/command-objects/security-group.rst

@@ -2,6 +2,10 @@
 security group
 ==============
 
+A **security group** acts as a virtual firewall for servers and other
+resources on a network. It is a container for security group rules
+which specify the network access rules.
+
 Compute v2, Network v2
 
 security group create

doc/source/command-objects/subnet-pool.rst

@@ -2,6 +2,9 @@
 subnet pool
 ===========
 
+A **subnet pool** contains a collection of prefixes in CIDR notation
+that are available for IP address allocation.
+
 Network v2
 
 subnet pool create

doc/source/command-objects/subnet.rst

@@ -2,6 +2,10 @@
 subnet
 ======
 
+A **subnet** is a block of IP addresses and associated configuration state.
+Subnets are used to allocate IP addresses when new ports are created on a
+network.
+
 Network v2
 
 subnet create