doc: Document factorial function

This has existed for a very long time, equivalent to the ! and !!
operators, but it was never documented.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/6ce1df0e-86a3-e544-743a-f357ff663f68%402ndquadrant.com

doc/src/sgml/func.sgml

@@ -1347,6 +1347,23 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>factorial</primary>
+        </indexterm>
+        <function>factorial</function> ( <type>bigint</type> )
+        <returnvalue>numeric</returnvalue>
+       </para>
+       <para>
+        Factorial
+       </para>
+       <para>
+        <literal>factorial(5)</literal>
+        <returnvalue>120</returnvalue>
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>