Refine the guidelines for rmgrdesc authors.

Refine the guidelines for rmgrdesc authors.

Clarify the goals of the recently added guidelines for rmgrdesc authors:
to avoid gratuitous inconsistencies across resource managers, and to
make it reasonably easy to write a reusable custom parser.

Beyond that, the guidelines leave rmgrdesc authors with a significant
amount of leeway.  This even includes the leeway to invent custom
conventions (in cases where it's warranted).

Follow-up to commit 7d8219a4.

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