Improve planner stats documentation

It was pointed out that in the planner stats documentation under
Extended Statistics, one of the sentences was a bit awkward.  Improve
that by rewording it slightly.

Discussion: https://postgr.es/m/154409976780.14137.2785644488950047100@wrigleys.postgresql.org

doc/src/sgml/perform.sgml

@@ -1086,8 +1086,8 @@ WHERE tablename = 'road';
    </para>
 
    <para>
-    Statistics objects are created using
-    <xref linkend="sql-createstatistics">, which see for more details.
+    Statistics objects are created using the
+    <xref linkend="sql-createstatistics"> command.
     Creation of such an object merely creates a catalog entry expressing
     interest in the statistics.  Actual data collection is performed
     by <command>ANALYZE</command> (either a manual command, or background