There were a few changes needed here:
1. There is no "API cell database", just the API
database, so this removes mentions of cells.
2. The VERSION argument was missing from the sync help.
3. The sync command does not create a database, it upgrades
the schema. Wording for that was borrowed from the
nova-manage db sync help.
4. Starting in Rocky, the api_db sync command also upgrades
the schema for the optional placement database if configured
so that's mentioned here as well.
Change-Id: Ibc49f93b8bd51d9a050acde5ef3dc8aad91321ca
Closes-Bug: #1778733
1476b030bd