doc: List compute_query_id in required config for pg_stat_statements

Not enabling compute_query_id would disable pg_stat_statements even if
the module is listed in shared_preload_libraries, so add it to the
minimum configuration set as listed in its documentation.

Author: Greg Nancarrow
Reviewed-by: Julien Rouhaud, Bharath Rupireddy
Discussion: https://postgr.es/m/CAJcOf-fXyb2QiDbwftD813UF70w-+BsK-03bFp1GrijXU9GQYQ@mail.gmail.com

doc/src/sgml/pgstatstatements.sgml

@@ -740,6 +740,7 @@
 # postgresql.conf
 shared_preload_libraries = 'pg_stat_statements'
 
+compute_query_id = on
 pg_stat_statements.max = 10000
 pg_stat_statements.track = all
 </programlisting>