diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 579ccd34d4..791f371d9d 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -2706,13 +2706,17 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
      <entry><structfield>blk_read_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry>Time spent reading data file blocks by backends in this database,
-      in milliseconds</entry>
+      in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
+      otherwise zero)
+     </entry>
     </row>
     <row>
      <entry><structfield>blk_write_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry>Time spent writing data file blocks by backends in this database,
-      in milliseconds</entry>
+      in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
+      otherwise zero)
+     </entry>
     </row>
     <row>
      <entry><structfield>stats_reset</structfield></entry>
diff --git a/doc/src/sgml/ref/explain.sgml b/doc/src/sgml/ref/explain.sgml
index c6f333c3c9..292c655b6b 100644
--- a/doc/src/sgml/ref/explain.sgml
+++ b/doc/src/sgml/ref/explain.sgml
@@ -171,8 +171,9 @@ ROLLBACK;
      <para>
       Include information on buffer usage. Specifically, include the number of
       shared blocks hit, read, dirtied, and written, the number of local blocks
-      hit, read, dirtied, and written, and the number of temp blocks read and
-      written.
+      hit, read, dirtied, and written, the number of temp blocks read and
+      written, and the time spent reading and writing data file blocks
+      (in milliseconds) if <xref linkend="guc-track-io-timing"/> is enabled.
       A <emphasis>hit</emphasis> means that a read was avoided because the block was
       found already in cache when needed.
       Shared blocks contain data from regular tables and indexes;