soft-anti-affinity known issue is out-of-date

In the Known Issues section of the OpenStack-specific Installer docs issues, there is a point about control plane anti-affinity.

The known issue has several problems:

• it is in the UPI section, when it is not a UPI-specific issue
• it mentions Control plane scale-out, when OCP only supports exactly 3 masters
• it is now possible to set anti-affinity from the install-config.yaml, and that should be the recommended solution when VM distribution across hosts is required.