doc: re-work west update to prep for new features

The west update docs mix together the steps for updating a project
with the set of projects that are being updated. This is inconvenient
now that west 0.9 has a concept of inactive projects.

Split the content into a part that describes *which* projects are
updated, and a separate part that describes how *each* project is
updated.

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