Improve documentation for pg_stat_checkpointer fields

pg_stat_checkpointer contains statistics for checkpoints and restartpoints.
Before 12915a5 documentation said only about checkpoints implying that
restartpoint is the variation of checkpoint.  12915a5 introduced
new separate statistics fields for restartpoints.  This commit explicitly
documents fields that are relevant for both checkpoints and restartpoints.

Reported-by: Magnus Hagander
Discussion: https://postgr.es/m/CABUevExav5-SR0x%2BG9kBUMV0G8XsvSUfuyyqmYBBJi6VHns6sw%40mail.gmail.com
Reviewed-by: Anton A. Melnikov

Author: akorotkov

doc/src/sgml/monitoring.sgml

@@ -3016,7 +3016,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para>
       <para>
        Total amount of time that has been spent in the portion of
-       checkpoint processing where files are written to disk, in milliseconds
+       processing checkpoints and restartpoints where files are written to disk,
+       in milliseconds
       </para></entry>
      </row>
 
@@ -3026,8 +3027,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para>
       <para>
        Total amount of time that has been spent in the portion of
-       checkpoint processing where files are synchronized to disk, in
-       milliseconds
+       processing checkpoints and restartpoints where files are synchronized to
+       disk, in milliseconds
       </para></entry>
      </row>
 
@@ -3036,7 +3037,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        <structfield>buffers_written</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of buffers written during checkpoints
+       Number of buffers written during checkpoints and restartpoints
       </para></entry>
      </row>
 

src/include/catalog/pg_proc.dat

@@ -5737,7 +5737,7 @@
   proparallel => 'r', prorettype => 'int8', proargtypes => '',
   prosrc => 'pg_stat_get_checkpointer_restartpoints_performed' },
 { oid => '2771',
-  descr => 'statistics: number of buffers written by the checkpointer',
+  descr => 'statistics: number of buffers written during checkpoints and restartpoints',
   proname => 'pg_stat_get_checkpointer_buffers_written', provolatile => 's',
   proparallel => 'r', prorettype => 'int8', proargtypes => '',
   prosrc => 'pg_stat_get_checkpointer_buffers_written' },
@@ -5760,12 +5760,12 @@
   proparallel => 'r', prorettype => 'timestamptz', proargtypes => '',
   prosrc => 'pg_stat_get_bgwriter_stat_reset_time' },
 { oid => '3160',
-  descr => 'statistics: checkpoint time spent writing buffers to disk, in milliseconds',
+  descr => 'statistics: checkpoint/restartpoint time spent writing buffers to disk, in milliseconds',
   proname => 'pg_stat_get_checkpointer_write_time', provolatile => 's',
   proparallel => 'r', prorettype => 'float8', proargtypes => '',
   prosrc => 'pg_stat_get_checkpointer_write_time' },
 { oid => '3161',
-  descr => 'statistics: checkpoint time spent synchronizing buffers to disk, in milliseconds',
+  descr => 'statistics: checkpoint/restartpoint time spent synchronizing buffers to disk, in milliseconds',
   proname => 'pg_stat_get_checkpointer_sync_time', provolatile => 's',
   proparallel => 'r', prorettype => 'float8', proargtypes => '',
   prosrc => 'pg_stat_get_checkpointer_sync_time' },