Bug#687900: document multiarch

[Jonathan Nieder <jrnieder@gmail.com>]

Russ Allbery wrote:

> There are various bugs already filed about some edge cases and
> specific issues with multiarch, but none to track the general
> documentation of multiarch handling in Policy.  This bug will be
> used to discuss the overall wording.

To be precise, am I correct in assuming this means documenting the
following?

 * the Multi-Arch field (meaning of values "same", "foreign",
   "allowed")
 * how the Architecture field affects dependencies
 * installation, configuration, upgrade, and removal procedure
   when a package is installed for more than one architecture

Other multiarch-related topics (such as paths for installing
libraries) seem to be covered elsewhere.

Thanks,
Jonathan