[libvirt] clarify doc references to documentation

[Dave Allan <dallan redhat com>]

This patch is a v2 follwing up on:

http://www.redhat.com/archives/libvir-list/2012-February/msg01104.html

rebased to the current HEAD and incorporating the changes suggested by
Eric.  I've also made a couple of minor unrelated cleanups to the
affected sentences to make the whole thing read a bit better.

Dave