doc: Update info on information schema usage tables

doc: Update info on information schema usage tables

Commit f40c6969d0 added the information schema usage tables but added
documentation that they did not fully work yet.  Commit e717a9a18b
then added SQL-standard function bodies, which made the information
schema views fully functional, but it neglected to update the
documentation.  This is now done here.

Reported-by: Erki Eessaar <erki.eessaar@taltech.ee>
Reviewed-by: Erki Eessaar <erki.eessaar@taltech.ee>
Discussion: https://www.postgresql.org/message-id/flat/AM9PR01MB8268EC7B696F9FE346CA5B93FEB8A%40AM9PR01MB8268.eurprd01.prod.exchangelabs.com