pgstat: consistent function comment formatting.

pgstat: consistent function comment formatting.

There was a wild mishmash of function comment formatting in pgstat, making it
hard to know what to use for any new function and hard to extend existing
comments (particularly due to randomly different forms of indentation).

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220329191727.mzzwbl7udhpq7pmf@alap3.anarazel.de
Discussion: https://postgr.es/m/20220308205351.2xcn6k4x5yivcxyd@alap3.anarazel.de