Update CREATE DATABASE documentation for deprecated IF NOT EXISTS feature

[jsternberg]

In 0.10, the IF NOT EXISTS syntax is now ignored when creating a database and it is the default and only behavior. We will be deprecating the syntax in 0.12 (from influxdata/influxdb#5707), but we need to include a deprecation notice and update the documentation for the change in behavior so confusions like influxdata/influxdb#5563 don't happen.