Description of problem:
 Libvirt should provide more accurate help messages of iface/net/pool-define in man vish and virsh help
Version-Release number of selected component (if applicable):
libvirt-1.2.6-1.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
<1> For iface-define:
[root@localhost network-scripts]# man virsh
       iface-define file
           Define a host interface from an XML file, the interface is just defined but not started.

[root@localhost network-scripts]# virsh help iface-define
  NAME
    iface-define - define (but don't start) a physical host interface from an XML file

  SYNOPSIS
    iface-define <file>

  DESCRIPTION
    Define a physical host interface.

  OPTIONS
    [--file] <string>  file containing an XML interface description

<2> For pool-define:
[root@localhost ~]# man virsh
pool-define file
           Create, but do not start, a pool object from the XML file.

[root@localhost ~]# virsh help pool-define
  NAME
    pool-define - define (but don't start) a pool from an XML file

  SYNOPSIS
    pool-define <file>

  DESCRIPTION
    Define a pool.

  OPTIONS
    [--file] <string>  file containing an XML pool description

<3> For net-define:
[root@localhost ~]# man virsh
net-define file
           Define a persistent virtual network from an XML file, the network is just defined but not instantiated (started).

[root@localhost ~]# virsh help net-define
  NAME
    net-define - define (but don't start) a network from an XML file

  SYNOPSIS
    net-define <file>

  DESCRIPTION
    Define a network.

  OPTIONS
    [--file] <string>  file containing an XML network description


Actual results:
As shown above help message, for example:

iface-define have a default function to update an existing physical host interface.

[root@localhost network-scripts]# virsh iface-list --all
Name                 State      MAC Address
--------------------------------------------
enp2s0               active     00:1b:21:27:4e:ce

[root@localhost network-scripts]# virsh iface-dumpxml enp2s0 --inactive
<interface type='ethernet' name='enp2s0'>
  <start mode='onboot'/>
  <protocol family='ipv4'>
    <ip address='10.66.70.108' prefix='23'/>
  </protocol>
</interface>

[root@localhost network-scripts]# cat enp2s0.xml
<interface type='ethernet' name='enp2s0'>
  <start mode='onboot'/>
  <mtu size='1500'/>
  <protocol family='ipv4'>
    <ip address='10.66.70.108' prefix='23'/>
  </protocol>
</interface>

[root@localhost network-scripts]# virsh iface-define enp2s0.xml
Interface enp2s0 defined from enp2s0.xml

[root@localhost network-scripts]# virsh iface-dumpxml enp2s0 --inactive
<interface type='ethernet' name='enp2s0'>
  <start mode='onboot'/>
  <mtu size='1500'/>
  <protocol family='ipv4'>
    <ip address='10.66.70.108' prefix='23'/>
  </protocol>
</interface>

Aslo, on net and pool, define command can update a net or pool with the same name and UUID exists.

Expected results:
For example on iface-define 
In man virsh
       iface-define file
           Define a host interface from an XML file, the interface is just defined but not started, or update an existing one.

In virsh help
 NAME
    iface-define - define (but don't start) or update a physical host interface from an XML file

  DESCRIPTION
    Define a physical host interface or update an existing one.

For net/pool-define should have similar updates.