Mention statistics objects in maintenance.sgml

The docs mentioned expression indexes as a way to improve selectivity
estimates for functions, but we have a second option to improve that by
creating extended statistics. So mention that too.

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/20210505210947.GA27406%40telsasoft.com

doc/src/sgml/maintenance.sgml

@@ -330,7 +330,8 @@
 
     <para>
      Also, by default there is limited information available about
-     the selectivity of functions.  However, if you create an expression
+     the selectivity of functions.  However, if you create a statistics
+     object or an expression
      index that uses a function call, useful statistics will be
      gathered about the function, which can greatly improve query
      plans that use the expression index.