Fix flags of some GUCs and improve some descriptions

Fix flags of some GUCs and improve some descriptions

This commit fixes some issues with GUCs:
- enable_incremental_sort was not marked as GUC_EXPLAIN, causing it to
not be listed in the output of EXPLAIN (SETTINGS) if using a value
different than the default, contrary to the other planner-level GUCs.
- trace_recovery_messages missed GUC_NOT_IN_SAMPLE, like the other
developer options.
- ssl_renegotiation_limit should be marked as COMPAT_OPTIONS_PREVIOUS.

While on it, this fixes one incorrect comment related to
autovacuum_freeze_max_age, and improves the descriptions of some other
GUCs, recently introduced.

Extracted from a larger patch set by the same author.

Author: Justin Pryzby
Description: https://postgr.es/m/20211129030833.GJ17618@telsasoft.com