Bug#764026: installation-guide: update manual for the desktop changes

[Cyril Brulebois <kibi@debian.org>]

Package: installation-guide
Version: 20140916
Severity: important

Hi,

so boot-time desktop selection got removed, replaced with a list of
desktops that can be picked when tasksel runs. It should be noted
that multiple desktops can be selected. I don't think it's going to
be possible to have all of them co-installable (all selected), so it
might be worth mentioning this might not work for each and every
combination of desktops.

It might be noted as well that manually passing desktop=foo at boot
time is still supported.

Mraw,
KiBi.