pgstat: remove some superflous comments from pgstat.h.

These would all need to be rephrased when moving to shared memory stats, but
since they don't provide actual information right now, remove them instead.

The comments for PgStat_Msg* are left in, because they will all be removed as
part of the shared memory stats patch.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

Author: anarazel

src/include/pgstat.h

@@ -776,9 +776,6 @@ typedef union PgStat_Msg
 
 #define PGSTAT_FILE_FORMAT_ID	0x01A5BCA6
 
-/*
- * Archiver statistics kept in the stats collector
- */
 typedef struct PgStat_ArchiverStats
 {
 	PgStat_Counter archived_count;	/* archival successes */
@@ -792,9 +789,6 @@ typedef struct PgStat_ArchiverStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_ArchiverStats;
 
-/*
- * Background writer statistics kept in the stats collector
- */
 typedef struct PgStat_BgWriterStats
 {
 	PgStat_Counter buf_written_clean;
@@ -803,9 +797,6 @@ typedef struct PgStat_BgWriterStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_BgWriterStats;
 
-/*
- * Checkpointer statistics kept in the stats collector
- */
 typedef struct PgStat_CheckpointerStats
 {
 	TimestampTz stats_timestamp;	/* time of stats file update */
@@ -818,10 +809,6 @@ typedef struct PgStat_CheckpointerStats
 	PgStat_Counter buf_fsync_backend;
 } PgStat_CheckpointerStats;
 
-/* ----------
- * PgStat_StatDBEntry			The collector's data per database
- * ----------
- */
 typedef struct PgStat_StatDBEntry
 {
 	Oid			databaseid;
@@ -866,10 +853,6 @@ typedef struct PgStat_StatDBEntry
 	HTAB	   *functions;
 } PgStat_StatDBEntry;
 
-/* ----------
- * PgStat_StatFuncEntry			The collector's data per function
- * ----------
- */
 typedef struct PgStat_StatFuncEntry
 {
 	Oid			functionid;
@@ -880,9 +863,6 @@ typedef struct PgStat_StatFuncEntry
 	PgStat_Counter f_self_time;
 } PgStat_StatFuncEntry;
 
-/*
- * Global statistics kept in the stats collector
- */
 typedef struct PgStat_GlobalStats
 {
 	TimestampTz stats_timestamp;	/* time of stats file update */
@@ -891,9 +871,6 @@ typedef struct PgStat_GlobalStats
 	PgStat_BgWriterStats bgwriter;
 } PgStat_GlobalStats;
 
-/*
- * Replication slot statistics kept in the stats collector
- */
 typedef struct PgStat_StatReplSlotEntry
 {
 	NameData	slotname;
@@ -908,9 +885,6 @@ typedef struct PgStat_StatReplSlotEntry
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatReplSlotEntry;
 
-/*
- * SLRU statistics kept in the stats collector
- */
 typedef struct PgStat_SLRUStats
 {
 	PgStat_Counter blocks_zeroed;
@@ -923,9 +897,6 @@ typedef struct PgStat_SLRUStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_SLRUStats;
 
-/*
- * Subscription statistics kept in the stats collector.
- */
 typedef struct PgStat_StatSubEntry
 {
 	Oid			subid;			/* hash key (must be first) */
@@ -935,10 +906,6 @@ typedef struct PgStat_StatSubEntry
 	TimestampTz stat_reset_timestamp;
 } PgStat_StatSubEntry;
 
-/* ----------
- * PgStat_StatTabEntry			The collector's data per table (or index)
- * ----------
- */
 typedef struct PgStat_StatTabEntry
 {
 	Oid			tableid;
@@ -971,9 +938,6 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter autovac_analyze_count;
 } PgStat_StatTabEntry;
 
-/*
- * WAL statistics kept in the stats collector
- */
 typedef struct PgStat_WalStats
 {
 	PgStat_Counter wal_records;