extended stats: Clarify behavior of omitting stat type clause

Pointed out by Jeff Janes
Discussion: https://postgr.es/m/CAMkU=1zGhK-nW10RAXhokcT3MM=YBg=j5LkG9RMDwmu3i0H0Og@mail.gmail.com

Author: alvherre

doc/src/sgml/ref/create_statistics.sgml

@@ -84,6 +84,8 @@ CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="PARAMETER">statistics_na
       <literal>ndistinct</literal>, which enables n-distinct statistics, and
       <literal>dependencies</literal>, which enables functional
       dependency statistics.
+      If this clause is omitted, all supported statistic types are
+      included in the statistics object.
       For more information, see <xref linkend="planner-stats-extended">
       and <xref linkend="multivariate-statistics-examples">.
      </para>