Some light copy cleanup for privatelink documentation based on feedback.
Fixes PRODSEC-232
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## Summary by CodeRabbit
* **Documentation**
* Updated PrivateLink guidance with revised architecture, supported
ports, routing, and security group instructions.
* Added labeled connectivity tests for direct PostgreSQL and PgBouncer
connections.
* Added connection-string examples for both connection methods.
* Replaced public-connectivity instructions with optional network
restriction steps and reorganized limitations guidance.
* Refined PrivateLink availability wording across platform security
documentation.
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