pgsql: 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(at)bowt(dot)ie>
Reviewed-By: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAH2-WzkbYuvwYKm-Y-72QEh6SPMQcAo9uONv+mR3bMGcu9E_Cg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c03c2eae0acbb82db8e626f71f367ef4b043d27d

Modified Files
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src/backend/access/rmgrdesc/rmgrdesc_utils.c | 20 -------------
src/include/access/rmgrdesc_utils.h | 44 ++++++++++++++++++++++++++++
2 files changed, 44 insertions(+), 20 deletions(-)