Add required `relation_name` or `relation_regex` note to Postgres config docs

[lediur]

What does this PR do?

Add some docs that either relation_name or relation_regex are required keys in the relations configuration object for the DataDog Postgres integration.

Motivation

Discovered that this property was required after encountering configuration errors after deployment. Adding this info to the docs may help others avoid this fate.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached
• [ ] If the PR doesn't need to be tested during QA, please add a qa/skip-qa label.