ebuild-writing/functions: Don't allow network access

There have been a number of bugs filed to remove network access from
ebuild phases ([1][],[2][],...), but until now this policy was undocumented
here.  We'll have to relax this restriction if the src_fetch ([3][],[4][],[5][]) or
pkg_fetch ([6][]) proposals being discussed for EAPI-6 are accepted, but
neither is currently mentioned in either the PMS (7) or Portage (8).

Yesterday on #gentoo-dev-help:

  16:12 < wking> Can anyone point me to the policy docs for not
    allowing network access (outside of SRC_URI and similar, e.g. from
    src_compile) in ebuilds?  I understand the reason for this, but
    I'd like to link to the official policy from a commit message in
    my overlay.
  16:26 <@mrueg> wking: i think it is just missing here:
    https://devmanual.gentoo.org/ebuild-writing/functions/src_test/index.html
  16:28 < wking> mrueg: so it's not explicitly documented yet?
  16:29 <@mrueg> wking: at least i haven't found it 

And in #gentoo-python:

  16:40 < idella4> prometheanfire: where the **** does it say in the
    manual no net rule for beyond src_fetch?????
  16:40 < jbergstroem> idella4: do you mean as in no downloading
    stuff? That's been around forever. lemme see if i can find a doc
    reference about it
  16:40 < jbergstroem> idella4: there's been a lot of bugs filed
  16:42 < idella4> thanks jberg
  16:42 < idella4> yes that's the one
  16:43 < jbergstroem> nor sure why it isn't; but it should probably live here 
    https://devmanual.gentoo.org/ebuild-writing/functions/index.html
  16:44 < jbergstroem> and also refr to pkg_fetch
  …
  19:39 < jbergstroem> wking: the devmanual could still benefit from
    being clearer about how/when to fetch sources.  Especially since
    newer packages try to automate suff like this (invoking wget
    from make or whatever for their deps)

[1]: https://bugs.gentoo.org/show_bug.cgi?id=315403
[2]: https://bugs.gentoo.org/show_bug.cgi?id=336771
[3]: https://bugs.gentoo.org/show_bug.cgi?id=182028
[4]: http://thread.gmane.org/gmane.linux.gentoo.devel/47645/focus=52918
[5]: http://thread.gmane.org/gmane.linux.gentoo.devel/87869
[6]: http://thread.gmane.org/gmane.linux.gentoo.devel/87869/focus=878773
7: git://git.overlays.gentoo.org/proj/pms.git
8: git://git.overlays.gentoo.org/proj/portage.git