Clarify firewall test and fix postgresql test command

What does this MR do?

Fixes an error introduced in https://gitlab.com/gitlab-org/gitlab-ee/commit/728029bc67bddb36badd963fdcdf45279261c838 and moves the point about firewalling to be inline with the command that tests whether the secondary can connect to the primary.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Documentation is currently broken

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4150 (closed)