add missing article;

All emulated networking interfaces are taken over by xnf(4) now

This follows the best practice and was brought to our attention by
Wei Liu <wei ! liu2 at citrix ! com>, thanks! Implementation idea
from reyk@.

Drop implementation notes section

slightly shorter Nd; ok mikeb

s/CAVEATS/IMPLEMENTATION NOTES/ to pacify mandoc lint; suggested by reyk@

Xen Netfront man page