Properly document that NEW is unassigned in plpgsql for DELETE (not NULL), and OLD is unassigned for INSERT, and NEW/OLD are unassigned (not NULL) for statement-level triggers.
Properly document that NEW is unassigned in plpgsql for DELETE (not
NULL), and OLD is unassigned for INSERT, and NEW/OLD are unassigned (not
NULL) for statement-level triggers.

Per report from Pavel Stehule

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