Bug#687900: document multiarch

[Francois Gouget <fgouget@free.fr>]

Also note that currently what little multiarch documentation seems to 
be out of date. In particular it says:

https://wiki.ubuntu.com/MultiarchSpec#Extended_semantics_of_per-architecture_package_relationships

| This introduces a new dependency syntax using the colon (:) character, 
| which is disallowed in package names, followed by the special value 
| any. No other values following the colon, including values matching 
| known architectures, are addressed by this implementation, and 
| behavior when encountering such values in a binary package's control 
| file is undefined. (It is recommended that dpkg and apt treat other 
| such package relationships as unsatisfiable.) This means that 
| declaring relationships on packages of a specific foreign architecture 
| is currently unsupported.


However support for architecture-specific dependency qualifiers was 
introduced into dpkg 1.16.5 (so before Debian 7):

* Bug #676232: dpkg: Please add support for specific arch qualifiers (cross-dependencies)
  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=676232

* dpkg commit 7acf7afa581457bc2e9bd6cd1f442e6f45c1db29
  Author: Thibaut Girka <thib@sitedethib.com>
  Date:   Thu Jun 28 01:11:11 2012 +0200

  Add support for specific arch-qualified dependencies
    
  This allows to use dependencies with arch-qualified package names. For
  example:

    Depends: libc6:amd64 (>= 2.14), libgcc1

  Closes: #676232

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


So it's no accident and should be documented.

-- 
Francois Gouget <fgouget@free.fr>              http://fgouget.free.fr/
      Sufficently advanced incompetence is indistinguishable from malice.