Fix nbtree posting list update desc output.

We cannot use the generic array_desc approach with per-tuple nbtree
posting list update metadata because array_desc can only deal with fixed
width elements (e.g., page offset numbers).  Using array_desc led to
incorrect rmgr descriptions for updates from nbtree DELETE/VACUUM WAL
records.

To fix, add specialized code to describe the update metadata as array
elements in desc output.  We now iterate over the update metadata using
an approach that matches related REDO routines.

Also stop showing the updates offset number array separately in nbtree
DELETE/VACUUM desc output.  It's redundant information, since the same
page offset numbers appear in the description of each individual update
element.  Also make some small tweaks to the way that we format arrays
in all desc routines (not just nbtree desc routines) to make arrays a
little less verbose.

Oversight in commit 1c453cfd, which enhanced the nbtree rmgr desc
routines.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkbYuvwYKm-Y-72QEh6SPMQcAo9uONv+mR3bMGcu9E_Cg@mail.gmail.com
