boards: Fixes and updates to chromebook docs

  - Fix some typos.

  - Update the docs about flashing and the debugging interfaces to
    reflect the latest changes in the internal collabora-lava-setup
    docs.

  - Add device-specific info about the actual devices present in the
    Collabora LAVA lab.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>