virsh: clean up attach-interface paragraph in man page

This makes the paragaph about attach-interface more descriptive and
correct, adding in a few bits of information that were previously
missing, e.g. --script is only allowed for bridge interfaces of Xen
domains, target name is regenerated if it starts with vnet, mac
address will be autogenerated if not specified.

(I did this in response to an email asking why a script couldn't be
specified for a bridge interface of a qemu domain, and why an
interface of type='ethernet' couldn't be created with
attach-interface)

tools/virsh.pod

@@ -2202,19 +2202,29 @@ Likewise, I<--shareable> is an alias for I<--mode shareable>.
 [I<--target target>] [I<--mac mac>] [I<--script script>] [I<--model model>]
 [I<--config>] [I<--inbound average,peak,burst>] [I<--outbound average,peak,burst>]
 
-Attach a new network interface to the domain.  I<type> can be either I<network>
-to indicate a physical network device or I<bridge> to indicate a bridge to a
-device.  I<source> indicates the source device.  I<target> allows to indicate
-the target device in the guest. Names starting with 'vnet' are considered as
-auto-generated an hence blanked out.  I<mac> allows to specify the MAC address
-of the network interface.  I<script> allows to specify a path to a script
-handling a bridge instead of the default one.  I<model> allows to specify the
-model type.  I<inbound> and I<outbound> control the bandwidth of the interface.
-I<peak> and I<burst> are optional, so "average,peak", "average,,burst" and
-"average" are also legal. Values for I<average> and I<peak> are
-expressed in kilobytes per second, while I<burst> is expressed in kilobytes
-in a single burst at -I<peak> speed as described in the Network XML
-documentation at L<http://libvirt.org/formatnetwork.html#elementQoS>.
+Attach a new network interface to the domain.  I<type> can be either
+I<network> to indicate connection via a libvirt virtual network or
+I<bridge> to indicate connection via a bridge device on the host.
+I<source> indicates the source of the connection (either the name of a
+network, or of a bridge device).  I<target> is used to specify the
+tap/macvtap device to be used to connect the domain to the
+source. Names starting with 'vnet' are considered as auto-generated
+and are blanked out/regenerated each time the interface is attached.
+I<mac> specifies the MAC address of the network interface; if a MAC
+address is not given, a new address will be automatically generated
+(and stored in the persistent configuration if "--config" is given on
+the commandline).  I<script> is used to specify a path to a custom
+script to be called while attaching to a bridge - this will be called
+instead of the default script not in addition to it; --script is valid
+only for interfaces of type I<bridge> and only for Xen domains.
+I<model> specifies the network device model to be presented to the
+domain.  I<inbound> and I<outbound> control the bandwidth of the
+interface.  I<peak> and I<burst> are optional, so "average,peak",
+"average,,burst" and "average" are also legal. Values for I<average>
+and I<peak> are expressed in kilobytes per second, while I<burst> is
+expressed in kilobytes in a single burst at -I<peak> speed as
+described in the Network XML documentation at
+L<http://libvirt.org/formatnetwork.html#elementQoS>.
 
 If I<--live> is specified, affect a running domain.
 If I<--config> is specified, affect the next startup of a persistent domain.