Doc: use CURRENT_USER not USER in plpgsql trigger examples.

While these two built-in functions do exactly the same thing,
CURRENT_USER seems preferable to use in documentation examples.
It's easier to look up if the reader is unsure what it is.
Also, this puts these examples in sync with an adjacent example
that already used CURRENT_USER.

Per question from Kirk Parker.

Discussion: https://postgr.es/m/CANwZ8rmN_Eb0h0hoMRS8Feftaik0z89PxVsKg+cP+PctuOq=Qg@mail.gmail.com

doc/src/sgml/plpgsql.sgml

@@ -4150,11 +4150,11 @@ CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
         -- making use of the special variable TG_OP to work out the operation.
         --
         IF (TG_OP = 'DELETE') THEN
-            INSERT INTO emp_audit SELECT 'D', now(), user, OLD.*;
+            INSERT INTO emp_audit SELECT 'D', now(), current_user, OLD.*;
         ELSIF (TG_OP = 'UPDATE') THEN
-            INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*;
+            INSERT INTO emp_audit SELECT 'U', now(), current_user, NEW.*;
         ELSIF (TG_OP = 'INSERT') THEN
-            INSERT INTO emp_audit SELECT 'I', now(), user, NEW.*;
+            INSERT INTO emp_audit SELECT 'I', now(), current_user, NEW.*;
         END IF;
         RETURN NULL; -- result is ignored since this is an AFTER trigger
     END;
@@ -4220,20 +4220,20 @@ CREATE OR REPLACE FUNCTION update_emp_view() RETURNS TRIGGER AS $$
             IF NOT FOUND THEN RETURN NULL; END IF;
 
             OLD.last_updated = now();
-            INSERT INTO emp_audit VALUES('D', user, OLD.*);
+            INSERT INTO emp_audit VALUES('D', current_user, OLD.*);
             RETURN OLD;
         ELSIF (TG_OP = 'UPDATE') THEN
             UPDATE emp SET salary = NEW.salary WHERE empname = OLD.empname;
             IF NOT FOUND THEN RETURN NULL; END IF;
 
             NEW.last_updated = now();
-            INSERT INTO emp_audit VALUES('U', user, NEW.*);
+            INSERT INTO emp_audit VALUES('U', current_user, NEW.*);
             RETURN NEW;
         ELSIF (TG_OP = 'INSERT') THEN
             INSERT INTO emp VALUES(NEW.empname, NEW.salary);
 
             NEW.last_updated = now();
-            INSERT INTO emp_audit VALUES('I', user, NEW.*);
+            INSERT INTO emp_audit VALUES('I', current_user, NEW.*);
             RETURN NEW;
         END IF;
     END;
@@ -4448,13 +4448,13 @@ CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
         --
         IF (TG_OP = 'DELETE') THEN
             INSERT INTO emp_audit
-                SELECT 'D', now(), user, o.* FROM old_table o;
+                SELECT 'D', now(), current_user, o.* FROM old_table o;
         ELSIF (TG_OP = 'UPDATE') THEN
             INSERT INTO emp_audit
-                SELECT 'U', now(), user, n.* FROM new_table n;
+                SELECT 'U', now(), current_user, n.* FROM new_table n;
         ELSIF (TG_OP = 'INSERT') THEN
             INSERT INTO emp_audit
-                SELECT 'I', now(), user, n.* FROM new_table n;
+                SELECT 'I', now(), current_user, n.* FROM new_table n;
         END IF;
         RETURN NULL; -- result is ignored since this is an AFTER trigger
     END;