Bug#878967: debian-policy: clarify purpose of debian/changelog

[Ross Vandegrift <ross@kallisti.us>]

Package: debian-policy
Version: 4.1.1.1
Severity: normal

Section 4.4 explains quite a bit about debian/changelog, but doesn't really
explain its purpose.  I took its purpose to be recording history and driving
automation - which led to some mistakes that might've been avoided.  During a
recent thread on mentors [1], I learned that the purpose is to provide a human
readable list of changes between released versions of Debian.  This came as a
surprise.

I'm not sure how this should be worded best, but I found a number of the
comments on the thread to be helpful, especially [2].

Thanks,
Ross

[1] https://lists.debian.org/debian-mentors/2017/10/msg00145.html
[2] https://lists.debian.org/debian-mentors/2017/10/msg00178.html

-- System Information:
Debian Release: 9.1
  APT prefers stable
  APT policy: (500, 'stable'), (40, 'unstable'), (30, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-4-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages debian-policy depends on:
ii  libjs-sphinxdoc  1.4.9-2

debian-policy recommends no packages.

Versions of packages debian-policy suggests:
pn  doc-base  <none>