pgsql: 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

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1dac2dd6779a7a5b35bd222f74bee1fe8c6e560d

Modified Files
--------------
doc/src/sgml/information_schema.sgml | 30 ++++++++++++++++--------------
1 file changed, 16 insertions(+), 14 deletions(-)