Show more detail in heapam rmgr descriptions.

Show more detail in heapam rmgr descriptions.

Add helper functions that output arrays in a standard format, and use
the functions inside heapdesc routines.  This allows tools like
pg_walinspect to show a detailed description of the page offset number
arrays for records like PRUNE and VACUUM (unless there was an FPI).

Also document the conventions that desc routines should follow.  Only
the heapdesc routines follow the conventions for now, so they're just
guidelines for the time being.

Based on a suggestion from Andres Freund.

Author: Melanie Plageman <[email protected]>
Reviewed-By: Peter Geoghegan <[email protected]>
Discussion: https://postgr.es/m/flat/20230109215842.fktuhesvayno6o4g%40awork3.anarazel.de