Document the +really convention under version conventions as well.
Add the -0~debNuX convention and restructure the explanation of
stable update versions.  Add some pseudo-headings to break up the
long list of version conventions.  Note that the stable update and
backport update conventions can stack.

Add the most common and widely-followed version conventions.  This
is missing some common ones (+dfsg primarily), but they can be
added later.

Talking about version conventions requires talking about upstream
and about native and non-native packages, none of which were
previously defined.  Add a definition of upstream to the Definitions
section and of native and non-native to the Source packages section.

    Wording: Helmut Grohne <helmut@subdivi.de>
    Seconded: Russ Allbery <rra@debian.org>
    Seconded: Christoph Berg <myon@debian.org>
    Seconded: Sean Whitton <spwhitton@spwhitton.name>
    Closes: #983657

Vagrant Cascadian authored Nov 16, 2020 and Sean Whitton committed Nov 16, 2020

While a consistent time was returned, depending on the timezone
the package was built in can cause the date to vary:

  https://reproducible-builds.org/docs/source-date-epoch/

    Wording: Sean Whitton <spwhitton@spwhitton.name>
    Seconded: Mattia Rizzolo <mattia@debian.org>
    Seconded: Holger Levsen <holger@layer-acht.org>
    Closes: #971023

    Wording: Sean Whitton <spwhitton@spwhitton.name>
    Seconded: gregor herrmann <gregoa@debian.org>
    Seconded: Graham Inggs <ginggs@debian.org>
    Closes: #959909

Suggested-by: Guillem Jover <guillem@debian.org>

Suggested-by: Christoph Berg <myon@debian.org>

    Wording: Philipp Kern <pkern@debian.org>
    Seconded: Sean Whitton <spwhitton@spwhitton.name>
    Seconded: Russ Allbery <rra@debian.org>
    Closes: #949390

* Fix error in Perl example for the gain root command.  Thanks, Niels
  Thykier.  (Closes: #949007)