doc: west: fix group filter documentation

The docs about how this works are inconsistent. Fix it by
adjusting the docs so they all match the implementation and
are consistent with each other.

Earlier in this page:

    Manifest files which appear earlier in the import order have
    higher precedence and are therefore concatenated later into the
    final group-filter.

Where for "import order" we have:

    Importing is done in this order:

    1. Manifests from self-import are imported first.
    2. The top-level manifest file’s definitions are handled next.
    3. Manifests from import-1, …, import-N, are imported in that order.

Therefore, "filter-1 to filter-N" is the wrong precedence order. It
should be "filter-N to filter-1", so that filter-N is lower precedence
than filter-1. That is consistent with import-1 happening before
import-N.

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