doc: improve wording of the need for analyze of exp. indexes

This is a followup commit on 33702079.

Reported-by: Justin Pryzby

Discussion: https://postgr.es/m/20201112211143.GL30691@telsasoft.com

Backpatch-through: 9.5

doc/src/sgml/ref/create_index.sgml

@@ -752,7 +752,7 @@ Indexes:
    For new expression indexes, it is necessary to run <link
    linkend="sql-analyze"><command>ANALYZE</command></link> or wait for
    the <link linkend="autovacuum">autovacuum daemon</link> to analyze
-   the table to generate statistics about new expression indexes.
+   the table to generate statistics for these indexes.
   </para>
 
   <para>