Comment 2 for bug 1859128

I guess we've added extra mentions to that in docs:
https://review.opendev.org/#/c/702934/
https://review.opendev.org/#/c/701548/

Also upgrade guide itself has mention to launch playbook with placement_upgrade variable set. Also there was an extra improval of upgrade script after 20.0.0 (iirc it's already in for 20.0.1) - https://review.opendev.org/#/c/710786/1

If you feel that we still missing mentioning somewhere in docs, please let us know about it (or place a fix :))