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

doc/src/sgml/plpgsql.sgml

@@ -3403,7 +3403,7 @@ RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;
       <para>
        Data type <type>RECORD</type>; variable holding the new
        database row for <command>INSERT</>/<command>UPDATE</> operations in row-level
-       triggers. This variable is <symbol>NULL</symbol> in statement-level triggers
+       triggers. This variable is unassigned in statement-level triggers
        and for <command>DELETE</command> operations.
       </para>
      </listitem>
@@ -3415,7 +3415,7 @@ RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;
       <para>
        Data type <type>RECORD</type>; variable holding the old
        database row for <command>UPDATE</>/<command>DELETE</> operations in row-level
-       triggers. This variable is <symbol>NULL</symbol> in statement-level triggers
+       triggers. This variable is unassigned in statement-level triggers
        and for <command>INSERT</command> operations.
       </para>
      </listitem>