doc: Update pg_stat_statements about query ID calculation of utilities

Since 3db72eb, the calculation of the query ID hash for utilities is not
done based on the textual query strings, but on their internal Query
representation, meaning that there can be an overlap when they use
literal constants.  The documentation of pg_stat_statements was missing
a refresh about that.

Extracted from a larger patch by me.

Discussion: https://postgr.es/m/Y+MRdEq9W9XVa2AB@paquier.xyz

Author: michaelpq

doc/src/sgml/pgstatstatements.sgml

@@ -487,13 +487,13 @@
 
   <para>
    Plannable queries (that is, <command>SELECT</command>, <command>INSERT</command>,
-   <command>UPDATE</command>, <command>DELETE</command>, and <command>MERGE</command>) are combined into a single
+   <command>UPDATE</command>, <command>DELETE</command>, and <command>MERGE</command>)
+   and utility commands are combined into a single
    <structname>pg_stat_statements</structname> entry whenever they have identical query
    structures according to an internal hash calculation.  Typically, two
    queries will be considered the same for this purpose if they are
    semantically equivalent except for the values of literal constants
-   appearing in the query.  Utility commands (that is, all other commands)
-   are compared strictly on the basis of their textual query strings, however.
+   appearing in the query.
   </para>
 
   <note>