docs: minor fixes to domain interface documentation

docs: minor fixes to domain interface documentation

A few examples for <interface> had a type='direct' interface with no
sub-elements. This is not allowed - a type='direct' interface must
have at least a source element. (Most likely the example was copied
from the type='user' or type='ethernet' examples - they *do* allow an
instance with no sub-elements).

There was also one place that mistakenly used %lt; ... %gt; instead of
&lt; ... &gt; (for some reason, I make that typo all the time).