virsh: tweak help output for VSH_OT_DATA

virsh: tweak help output for VSH_OT_DATA

https://bugzilla.redhat.com/show_bug.cgi?id=609044 complained
that 'virsh help pool-create-as' didn't document the shortcut
that you can do 'virsh pool-create-as $name $type --target $target'
rather than having to supply the four optional source- arguments
in order to fill out the necessary positional arguments.

This one-liner changes the help output to hopefully make this more obvious:

  NAME
    pool-create-as - create a pool from a set of args

  SYNOPSIS
    pool-create-as <name> [--print-xml] <type> [<source-host>] [<source-path>] [<source-dev>] [<source-name>] [<target>] [--source-format <string>]

  DESCRIPTION
    Create a pool.

  OPTIONS
    [--name] <string>  name of the pool
    --print-xml      print XML document, but don't define/create
    [--type] <string>  type of the pool
    [--source-host] <string>  source-host for underlying storage
    [--source-path] <string>  source path for underlying storage
    [--source-dev] <string>  source device for underlying storage
    [--source-name] <string>  source name for underlying storage
    [--target] <string>  target for underlying storage
    --source-format <string>  format for underlying storage

* tools/virsh.c (vshCmddefHelp): Make it more obvious that data
arguments may, but not must, be specified by option leaders.