doc: Mention CASCADE/RESTRICT for DROP STATISTICS

This grammar has no effect as there are no dependencies on statistics,
but it is supported by the parser.  This is more consistent with the
other DROP commands.

Author: Vignesh C
Discussion: https://postgr.es/m/CALDaNm1LA=yNmzcSfy+0oe6CEAgsxXRf_-UutE3ZncFi8QkFNQ@mail.gmail.com
Backpatch-through: 10

doc/src/sgml/ref/drop_statistics.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-DROP STATISTICS [ IF EXISTS ] <replaceable class="parameter">name</replaceable> [, ...]
+DROP STATISTICS [ IF EXISTS ] <replaceable class="parameter">name</replaceable> [, ...] [ CASCADE | RESTRICT ]
 </synopsis>
  </refsynopsisdiv>
 
@@ -59,6 +59,18 @@ DROP STATISTICS [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>CASCADE</literal></term>
+    <term><literal>RESTRICT</literal></term>
+
+    <listitem>
+     <para>
+      These key words do not have any effect, since there are no dependencies
+      on statistics.
+     </para>
+    </listitem>
+   </varlistentry>
+
   </variablelist>
  </refsect1>