Document how to use config overrides for multivalued keys in ini files

[Adrien Cunin]

https://docs.openstack.org/project-deploy-guide/openstack-ansible/draft/app-advanced-config-override.html#overriding-conf-files

This doesn't explain what to do if I wanted to achieve the following in an ini configuration file:

  [section]
  key = value1
  key = value2

[Mohammed Naser (mnaser)]

fyi, multistropt can use commas instead

key = value1, value2