Postgres Password Rotation Trigger is not documented

### What problem are you facing?

The password rotation trigger feature introduced in #348 wasn't properly documented, and the only place so far where the behavior is explained remains the PR itself.

### How could Crossplane help solve your problem?

Add it to the examples/docs.

Start by reading PR #348, which contains the existing explanation of the password rotation trigger behavior. Locate the relevant examples/docs area and document that behavior there; done means users can find the trigger documentation without referring to the pull request.

Written by the indexing model from the issue text.