Docs: Fix postgres configuration in Geo HA

What does this MR do?

The docs need to specify adding these roles to the PG nodes because they have effects that are not captured elsewhere:

• geo_primary_role https://gitlab.com/gitlab-org/omnibus-gitlab/blob/12.2.5+ee.0/files%2Fgitlab-cookbooks%2Fpackage%2Flibraries%2Fconfig%2Froles%2Fgeo_primary.rb#L21-26
• geo_secondary_role https://gitlab.com/gitlab-org/omnibus-gitlab/blob/12.2.5+ee.0/files/gitlab-cookbooks/package/libraries/config/roles/geo_secondary.rb#L23-29

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
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