I can't see the option to Update to 20.04 from 19.04

The whole -d parameter thing has confused me too, but it turns out it is necessary to update to the LTS from 18.04 and 19.10 before Canonical enables the prompts in :update: Software Updater.

Looks like that command in the release notes should in fact include -d, at least for now.

       -d, --devel-release
              If  using  the latest supported release, upgrade to the develop‐
              ment release

See also: