diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index acc6e2bc31..49d4bb13b9 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -3390,7 +3390,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para>
       <para>
        Time spent reading data file blocks by backends in this database,
-       in milliseconds
+       in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
+       otherwise zero)
       </para></entry>
      </row>
 
@@ -3400,7 +3401,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para>
       <para>
        Time spent writing data file blocks by backends in this database,
-       in milliseconds
+       in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
+       otherwise zero)
       </para></entry>
      </row>
 
diff --git a/doc/src/sgml/ref/explain.sgml b/doc/src/sgml/ref/explain.sgml
index 7ddb5a05c6..1c19e254dc 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;