Geo: Improve non-Omnibus database documentation

What does this MR do?

• Be specific about Geo non-Omnibus database support in docs.
• Clarify Geo + external PostgreSQL instances cases
• Accommodate the possibility of using AWS RDS replication

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8221

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards