Clarify md5_auth_* usage for Geo primary site

What does this MR do and why?

This spawned from an internal Slack discussion - the text vs the example below are inconsistent.

This should be primary + secondary IPs, not just secondary, otherwise this happens:

/opt/gitlab/embedded/lib/ruby/gems/2.7.0/gems/activerecord-6.1.4.7/lib/active_record/connection_adapters/postgresql_adapter.rb:81:in `rescue in new_client': FATAL:  no pg_hba.conf entry for host "172.31.17.189", user "gitlab", database "gitlabhq_production", SSL on (ActiveRecord::NoDatabaseError)
FATAL:  no pg_hba.conf entry for host "172.31.17.189", user "gitlab", database "gitlabhq_production", SSL off

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @jmarquez2 @calebcooper