update cobra docs: specify the vitess directory and the right version label

We currently recommend people to run `v23.0.0:23.0` when asking users to re-gen the cobra docs. We should use the release branch instead of the tag because the tag does not exist yet, e.g.: `release-23.0:23.0`.

We should also specify the `VITESS_DIR` in the env variables to export. Right now we only ask to export the `COBRADOC...` variable.