doc: Fix statement about ON CONFLICT and deferrable constraints.

The description of deferrable constraints in create_table.sgml states
that deferrable constraints cannot be used as conflict arbitrators in
an INSERT with an ON CONFLICT DO UPDATE clause, but in fact this
restriction applies to all ON CONFLICT clauses, not just those with DO
UPDATE. Fix this, and while at it, change the word "arbitrators" to
"arbiters", to match the terminology used elsewhere.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/CAEZATCWsybvZP3ce8rGcVNx-QHuDOJZDz8y=p1SzqHwjRXyV4Q@mail.gmail.com
Backpatch-through: 14

doc/src/sgml/ref/create_table.sgml

@@ -1274,8 +1274,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       <literal>REFERENCES</literal> (foreign key) constraints accept this
       clause.  <literal>NOT NULL</literal> and <literal>CHECK</literal> constraints are not
       deferrable.  Note that deferrable constraints cannot be used as
-      conflict arbitrators in an <command>INSERT</command> statement that
-      includes an <literal>ON CONFLICT DO UPDATE</literal> clause.
+      conflict arbiters in an <command>INSERT</command> statement that
+      includes an <literal>ON CONFLICT</literal> clause.
      </para>
     </listitem>
    </varlistentry>