doc: west: document west 0.10 group-filter + import behavior

The west 0.9 documentation for group-filter states:

    Only the top level manifest file’s manifest: group-filter:
    value has any effect. The manifest: group-filter: values in any
    imported manifests are ignored.

This turns out to have been a mistake, because all users who import a
manifest which makes some projects inactive will have to also manually
disable the same groups just to get the same default list of projects.

Example manifest fragments showing the issue:

   # my/west.yml
   manifest:
     projects:
       - name: upstream
         import: true

and:

   # upstream/west.yml
   manifest:
     group-filter: [-disabled-group]
     projects:
       - name: i-want-this
       - name: i-do-not-want-this
         groups:
         - disabled-group

As written, my/west.yml will include 'i-do-not-want-this' as an
active project.

This kind of leaky faucet has proven to be an unintuitive and poor
user experience. Users expect to get the default projects without
having to copy the group-filter for everything they import.

We can't reverse this behavior without pushing out a new schema
version, however: manifest schema 0.9, and therefore west 0.9.x, is
doomed to this bad behavior.

What we'll do to move past this is create a new schema version 0.10
that does the right thing, document the 0.10 behavior, and discourage
combining these two features in west 0.9.x.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>