Docs for updated gke_bootstrap_script.sh
The zone variable information in the table is not correct. The default zone is us-central1-b, not us-central1-a. The ZONE variable does not appear to be overridable, it is the concatenation of $REGION and $ZONE_EXTENSION. Add row for ZONE_EXTENSION since this and REGION are what must be specified to override the defaults.
This is updating the docs to be in line with !228 (merged) to the script.