Bug#865120: release-notes: document i386/amd64 kernel changes for jessie->stretch

[Stuart Prescott <stuart@debian.org>]

Package: release-notes
Severity: important

With Stretch, amd64 flavour kernels are no longer supplied within the i386
Debian architecture (i.e. for a 64 bit kernel with a 32 bit userspace). The
release notes should document that these kernel packages are no longer
offered so that users don't accidentally run unsupported kernels indefinitely
(hence severity:important).

The upgrade path is to enable amd64 as a foreign architecture and then install
the relevant amd64 kernel image.

A 0th draft at some wording for §4.6/4.6.1:

Users of the i386 port are advised that the amd64 flavour of kernel
is no longer included within the i386 port but that these kernels
should instead be installed directly from the amd64 port using the
ability for apt and dpkg to install foreign architecture packages
(known as multiarch).

- check if running i386 userspace with amd64 kernel:
    `dpkg --print-architure` is i386, and
    `uname -r` ends in amd64

- add amd64 as foreign architecture
    `dpkg --add-architecture amd64; apt-get update`

- (install the appropriate kernel metapackage as described already)

cheers
Stuart