doc: Clarify RLS policies applied for ON CONFLICT DO NOTHING.

On the CREATE POLICY page, the description of per-command policies
stated that SELECT policies are applied when an INSERT has an ON
CONFLICT DO NOTHING clause. However, that is only the case if it
includes an arbiter clause, so clarify that.

While at it, also clarify the comment in the regression tests that
cover this.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Viktor Holmberg <v@viktorh.net>
Discussion: https://postgr.es/m/CAEZATCXGwMQ+x00YY9XYG46T0kCajH=21QaYL9Xatz0dLKii+g@mail.gmail.com
Backpatch-through: 14

doc/src/sgml/ref/create_policy.sgml

@@ -294,7 +294,9 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
        </para>
        <para>
          If an <literal>INSERT</literal> has an <literal>ON CONFLICT DO
-         NOTHING/UPDATE</literal> clause, <literal>SELECT</literal>
+         UPDATE</literal> clause, or an <literal>ON CONFLICT DO
+         NOTHING</literal> clause with an arbiter index or constraint
+         specification, then <literal>SELECT</literal>
          permissions are required on the relation, and the rows proposed for
          insertion are checked using the relation's <literal>SELECT</literal>
          policies.  If a row proposed for insertion does not satisfy the
@@ -540,7 +542,11 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       <row>
        <entry><command>INSERT ... ON CONFLICT</command></entry>
        <entry>
-        Check new row&nbsp;<footnote id="rls-on-conflict-priv">
+        Check new row&nbsp;<footnote>
+         <para>
+          If an arbiter index or constraint is specified.
+         </para>
+        </footnote><footnote id="rls-on-conflict-priv">
          <para>
           Row proposed for insertion is checked regardless of whether or not a
           conflict occurs.