Policy documentation improvements

In ALTER POLICY, use 'check_expression' instead of 'expression' for the
parameter, to match up with the recent CREATE POLICY change.

In CREATE POLICY, frame the discussion as granting access to rows
instead of limiting access to rows.  Further, clarify that the
expression must return true for rows to be visible/allowed and that a
false or NULL result will mean the row is not visible/allowed.

Per discussion with Dean Rasheed and Robert.

doc/src/sgml/ref/alter_policy.sgml

@@ -24,7 +24,7 @@ PostgreSQL documentation
 ALTER POLICY <replaceable class="parameter">name</replaceable> ON <replaceable class="parameter">table_name</replaceable>
     [ RENAME TO <replaceable class="PARAMETER">new_name</replaceable> ]
     [ 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>
@@ -87,7 +87,7 @@ ALTER POLICY <replaceable class="parameter">name</replaceable> ON <replaceable c
    </varlistentry>
 
    <varlistentry>
-    <term><replaceable class="parameter">expression</replaceable></term>
+    <term><replaceable class="parameter">using_expression</replaceable></term>
     <listitem>
      <para>
       The USING expression for the policy.  This expression will be added as a

doc/src/sgml/ref/create_policy.sgml

@@ -39,14 +39,21 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
   </para>
 
   <para>
-   A policy limits the ability to SELECT, INSERT, UPDATE, or DELETE rows
-   in a table to those rows which match the relevant policy expression.
-   Existing table rows are checked against the expression specified via
-   USING, while new rows that would be created via INSERT or UPDATE are
-   checked against the expression specified via WITH CHECK.  Generally,
-   the system will enforce filter conditions imposed using security
-   policies prior to qualifications that appear in the query itself, in
-   order to the prevent the inadvertent exposure of the protected data to
+   A policy grants the ability to SELECT, INSERT, UPDATE, or DELETE rows
+   which match the relevant policy expression.  Existing table rows are
+   checked against the expression specified via USING, while new rows that
+   would be created via INSERT or UPDATE are checked against the expression
+   specified via WITH CHECK.  When a USING expression returns true for a given
+   row then that row is visible to the user, while if a false or null is
+   returned then the row is not visible.  When a WITH CHECK expression
+   returns true for a row then that row is added, while if a false or null is
+   returned then an error occurs.
+  </para>
+
+  <para>
+   Generally, the system will enforce filter conditions imposed using
+   security policies prior to qualifications that appear in the query itself,
+   in order to the prevent the inadvertent exposure of the protected data to
    user-defined functions which might not be trustworthy.  However,
    functions and operators marked by the system (or the system
    administrator) as LEAKPROOF may be evaluated before policy
@@ -158,8 +165,10 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       Any <acronym>SQL</acronym> conditional expression (returning
       <type>boolean</type>).  The conditional expression cannot contain
       any aggregate or window functions.  This expression will be added
-      to queries that refer to the table if row level security is enabled,
-      and will allow access to rows matching the expression.
+      to queries that refer to the table if row level security is enabled
+      and rows for which the expression returns true will be visible.  Any
+      rows for which the expression returns false or null will not be
+      visible to the user.
      </para>
     </listitem>
    </varlistentry>
@@ -172,9 +181,10 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       <type>boolean</type>).  The conditional expression cannot contain
       any aggregate or window functions.  This expression will be used with
       <command>INSERT</command> and <command>UPDATE</command> queries against
-      the table if row level security is enabled and an error will be thrown
-      if the expression evaluates to false for any of the records inserted
-      or any of the records which result from the update.
+      the table if row level security is enabled and only rows where the
+      expression evaluates to true will be allowed.  An error will be thrown
+      if the expression evaluates to false or null for any of the records
+      inserted or any of the records which result from the update.
      </para>
     </listitem>
    </varlistentry>