docs: fix indentation of sub-elements of <ip> in network XML

docs: fix indentation of sub-elements of <ip> in network XML

The sub-elements of <ip> had been placed at the same level of
indentation as ip itself, implying that they were really elements of
<network>. Within that, sub-elements of ip/dhcp were also at that same
level. These have been double-indented.

At the same time, I realized that the documentation for the new <dns>
element had been placed right in the middle of the description of the
sub-elements of <ip>. I moved it up out of the way.