CREATE POLICY expression -> using_expression

The syntax for CREATE POLICY simply used "expression" for the USING
expression, while the WITH CHECK expression was "check_expression".
Given that we have two expressions, it's sensible to explcitly name both
to maintain clarity.

This patch simply changes the generic "expression" to be
"using_expression".

Pointed out by Peter Geoghegan.

Author: sfrost

doc/src/sgml/ref/create_policy.sgml

@@ -24,7 +24,7 @@ PostgreSQL documentation
 CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable class="parameter">table_name</replaceable>
     [ FOR { ALL | SELECT | INSERT | UPDATE | DELETE } ]
     [ TO { <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...] ]
-    [ USING ( <replaceable class="parameter">expression</replaceable> ) ]
+    [ USING ( <replaceable class="parameter">using_expression</replaceable> ) ]
     [ WITH CHECK ( <replaceable class="parameter">check_expression</replaceable> ) ]
 </synopsis>
  </refsynopsisdiv>
@@ -152,7 +152,7 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
    </varlistentry>
 
    <varlistentry>
-    <term><replaceable class="parameter">expression</replaceable></term>
+    <term><replaceable class="parameter">using_expression</replaceable></term>
     <listitem>
      <para>
       Any <acronym>SQL</acronym> conditional expression (returning