docs: fix alerting limitations for postgres

[mhristof]

I've created a couple of alerts using the postgres engine and the ones in table format seem to work and the ones in the time series format error out with NoData.

I could not find any references of this online - it would be great if anyone can verify this before updating the docs

What is this feature?

Update docs to show that time series alerts are not working and table alerts are.

Why do we need this feature?

Indescrepancy in the docs

Who is this feature for?

All postgres users

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective. (N/A)
• If this is a pre-GA feature, it is behind a feature toggle. (N/A)
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[CLAassistant]

All committers have signed the CLA.