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