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

src/backend/utils/misc/guc.c

@@ -1034,7 +1034,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_incremental_sort", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of incremental sort steps."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_incremental_sort,
 		true,
@@ -1223,7 +1224,7 @@ static struct config_bool ConfigureNamesBool[] =
 	},
 	{
 		{"ssl_passphrase_command_supports_reload", PGC_SIGHUP, CONN_AUTH_SSL,
-			gettext_noop("Also use ssl_passphrase_command during server reload."),
+			gettext_noop("Controls whether ssl_passphrase_command is called during server reload."),
 			NULL
 		},
 		&ssl_passphrase_command_supports_reload,
@@ -2944,8 +2945,8 @@ static struct config_int ConfigureNamesInt[] =
 
 	{
 		{"commit_siblings", PGC_USERSET, WAL_SETTINGS,
-			gettext_noop("Sets the minimum concurrent open transactions before performing "
+			gettext_noop("Sets the minimum number of concurrent open transactions "
-						 "commit_delay."),
+						 "required before performing commit_delay."),
 			NULL
 		},
 		&CommitSiblings,
@@ -3298,10 +3299,7 @@ static struct config_int ConfigureNamesInt[] =
 		},
 		&autovacuum_freeze_max_age,
 
-		/*
+		/* see vacuum_failsafe_age if you change the upper-limit value. */
-		 * see pg_resetwal and vacuum_failsafe_age if you change the
-		 * upper-limit value.
-		 */
 		200000000, 100000, 2000000000,
 		NULL, NULL, NULL
 	},
@@ -3403,7 +3401,7 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"ssl_renegotiation_limit", PGC_USERSET, CONN_AUTH_SSL,
+		{"ssl_renegotiation_limit", PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
 			gettext_noop("SSL renegotiation is no longer supported; this can only be 0."),
 			NULL,
 			GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE,
@@ -4673,7 +4671,7 @@ static struct config_enum ConfigureNamesEnum[] =
 
 	{
 		{"compute_query_id", PGC_SUSET, STATS_MONITORING,
-			gettext_noop("Compute query identifiers."),
+			gettext_noop("Enables in-core computation of query identifiers."),
 			NULL
 		},
 		&compute_query_id,
@@ -4837,7 +4835,8 @@ static struct config_enum ConfigureNamesEnum[] =
 		{"trace_recovery_messages", PGC_SIGHUP, DEVELOPER_OPTIONS,
 			gettext_noop("Enables logging of recovery-related debugging information."),
 			gettext_noop("Each level includes all the levels that follow it. The later"
-						 " the level, the fewer messages are sent.")
+						 " the level, the fewer messages are sent."),
+			GUC_NOT_IN_SAMPLE,
 		},
 		&trace_recovery_messages,