Add paragraph regarding packages without existing HOMEPAGE

See Dev-ML discussion: http://article.gmane.org/gmane.linux.gentoo.devel/92024
As proposed in https://bugs.gentoo.org/show_bug.cgi?id=517644#c6
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].

Portage *does* have a network-sandbox feature [9,10], which applies to
all src_* phases except src_unpack (for live ebuilds).  I think it
should be applied to pkg_* phases too, but Michał thinks they may need
networking "for some kind of IPC" [11].  Alexis pointed out that a
strict network-sandbox will break distcc [12], but since that network
access is the result of enabling a Portage feature (and not initiated
by the ebuild or package itself), it falls outside the scope of this
policy.

[1]: https://bugs.gentoo.org/show_bug.cgi?id=315403
     Summary: media-video/jubler fails to build without network
[2]: https://bugs.gentoo.org/show_bug.cgi?id=336771
     Summary: dev-libs/libevent tests require network access
[3]: https://bugs.gentoo.org/show_bug.cgi?id=182028
     Summary: [Future EAPI] About managing CVS/SUBVERSION version of
       software
[4]: http://thread.gmane.org/gmane.linux.gentoo.devel/47645/focus=52918
     From: Marijn Schouten <hkBst@gentoo.org>
     Subject: src_fetch (was Re: EAPI 1 (Was: Re: Monthly Gentoo
       Council Reminder for April))
     Date: 2007-11-09 17:41:38 GMT
[5]: http://thread.gmane.org/gmane.linux.gentoo.devel/87869
     From: Michał Górny <mgorny@gentoo.org>
     Subject: [EAPI 6] src_fetch() for fetching live sources
     Date: 2013-08-27 10:01:27 GMT
[6]: http://thread.gmane.org/gmane.linux.gentoo.devel/87869/focus=878773
     From: Ian Stakenvicius <axs@gentoo.org>
     Subject: Re: [EAPI 6] src_fetch() for fetching live sources
     Date: 2013-08-27 14:33:27 GMT
[7]: git://git.overlays.gentoo.org/proj/pms.git
[8]: git://git.overlays.gentoo.org/proj/portage.git
[9]: http://thread.gmane.org/gmane.linux.gentoo.devel/87641
     From: Michał Górny <mgorny@gentoo.org>
     Subject: New developer features in portage: cgroup,
       network-sandbox, ipc-sandbox
     Date: 2013-08-20 10:26:03 GMT
[10]: http://thread.gmane.org/gmane.linux.gentoo.devel/91131
      From: Michał Górny <mgorny@gentoo.org>
      Subject: RFC: enabling ipc-sandbox & network-sandbox by default
      Date: 2014-05-11 21:42:38 GMT
[11]: http://article.gmane.org/gmane.linux.gentoo.devel/87641
      From: Michał Górny <mgorny@gentoo.org>
      Subject: New developer features in portage: cgroup,
        network-sandbox, ipc-sandbox
      Date: 2013-08-20 10:26:03 GMT
[12]: http://thread.gmane.org/gmane.linux.gentoo.devel/87641/focus=87658
      From: Alexis Ballier <aballier@gentoo.org>
      Subject: Re: New developer features in portage: cgroup,
        network-sandbox, ipc-sandbox
      Date: 2013-08-20 15:04:35 GMT
ebuild-writing/common-mistakes: Mention the 'doc' USE flag

When reminding folks to install the docs, also remind them that they
should be using the 'doc' USE flag for serious docs.  This USE flag is
the standard way to enable/disable doc installation, and we don't want
ebuilds blindly pulling in doc depenencies regardless of how it's set.
Use some of Ulrich Müller's wording [1].

[1]: https://bugs.gentoo.org/show_bug.cgi?id=522800#c3

Thanks-to: Ulrich Müller <ulm@gentoo.org>
Merge pull request #17 from mrueg/master

 Add paragraph regarding packages without existing HOMEPAGE
Merge pull request #23 from wking/doc-use

ebuild-writing/common-mistakes: Mention the 'doc' USE flag
Merge pull request #21 from wking/no-network-access

ebuild-writing/functions: Don't allow network access
Document use dependency defaults on this section of the devmanual