Doc: add a summary table to the CREATE POLICY docs.

This table summarizes which RLS policy expressions apply to each
command type, and whether they apply to the old or new tuples (or
both), which saves reading through a lot of text.

Rod Taylor, hacked on by me. Reviewed by Fabien Coelho.

Discussion: https://postgr.es/m/CAHz80e4HxJShm6m9ZWFrHW=pgd2KP=RZmfFnEccujtPMiAOW5Q@mail.gmail.com

Author: deanrasheed

doc/src/sgml/ref/create_policy.sgml

@@ -72,7 +72,10 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
   <para>
    Policies can be applied for specific commands or for specific roles.  The
    default for newly created policies is that they apply for all commands and
-   roles, unless otherwise specified.
+   roles, unless otherwise specified.  Multiple policies may apply to a single
+   command; see below for more details.
+   <xref linkend="sql-createpolicy-summary"> summarizes how the different types
+   of policy apply to specific commands.
   </para>
 
   <para>
@@ -353,6 +356,105 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
      </varlistentry>
 
    </variablelist>
+
+   <table id="sql-createpolicy-summary">
+    <title>Policies Applied by Command Type</title>
+    <tgroup cols="6">
+     <colspec colnum="4" colname="update-using">
+     <colspec colnum="5" colname="update-check">
+     <spanspec namest="update-using" nameend="update-check" spanname="update">
+     <thead>
+      <row>
+       <entry morerows="1">Command</entry>
+       <entry><literal>SELECT/ALL policy</literal></entry>
+       <entry><literal>INSERT/ALL policy</literal></entry>
+       <entry spanname="update"><literal>UPDATE/ALL policy</literal></entry>
+       <entry><literal>DELETE/ALL policy</literal></entry>
+      </row>
+      <row>
+       <entry><literal>USING expression</literal></entry>
+       <entry><literal>WITH CHECK expression</literal></entry>
+       <entry><literal>USING expression</literal></entry>
+       <entry><literal>WITH CHECK expression</literal></entry>
+       <entry><literal>USING expression</literal></entry>
+      </row>
+     </thead>
+     <tbody>
+      <row>
+       <entry><command>SELECT</command></entry>
+       <entry>Existing row</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+      </row>
+      <row>
+       <entry><command>SELECT FOR UPDATE/SHARE</command></entry>
+       <entry>Existing row</entry>
+       <entry>&mdash;</entry>
+       <entry>Existing row</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+      </row>
+      <row>
+       <entry><command>INSERT</command></entry>
+       <entry>&mdash;</entry>
+       <entry>New row</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+      </row>
+      <row>
+       <entry><command>INSERT ... RETURNING</command></entry>
+       <entry>
+        New row
+        <footnote id="rls-select-priv">
+         <para>
+          If read access is required to the existing or new row (for example,
+          a <literal>WHERE</literal> or <literal>RETURNING</literal> clause
+          that refers to columns from the relation).
+         </para>
+        </footnote>
+       </entry>
+       <entry>New row</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+      </row>
+      <row>
+       <entry><command>UPDATE</command></entry>
+       <entry>
+        Existing &amp; new rows
+        <footnoteref linkend="rls-select-priv">
+       </entry>
+       <entry>&mdash;</entry>
+       <entry>Existing row</entry>
+       <entry>New row</entry>
+       <entry>&mdash;</entry>
+      </row>
+      <row>
+       <entry><command>DELETE</command></entry>
+       <entry>
+        Existing row
+        <footnoteref linkend="rls-select-priv">
+       </entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+       <entry>&mdash;</entry>
+       <entry>Existing row</entry>
+      </row>
+      <row>
+       <entry><command>ON CONFLICT DO UPDATE</command></entry>
+       <entry>Existing &amp; new rows</entry>
+       <entry>&mdash;</entry>
+       <entry>Existing row</entry>
+       <entry>New row</entry>
+       <entry>&mdash;</entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
   </refsect2>
 
   <refsect2>