Add explanations which are influenced by track_io_timing

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 579ccd34d4..186d1c10ca 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..c9fa7001c6 100644
--- a/doc/src/sgml/ref/explain.sgml
+++ b/doc/src/sgml/ref/explain.sgml
@@ -172,7 +172,8 @@ ROLLBACK;
       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.
+      written. In addition, If <xref linkend="guc-track-io-timing"/> is enabled,
+      also include I/O Timings.
       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;
@@ -184,6 +185,8 @@ ROLLBACK;
       number of blocks <emphasis>written</emphasis> indicates the number of
       previously-dirtied blocks evicted from cache by this backend during
       query processing.
+      <emphasis>I/O Timings</emphasis> indicates the total time spent reading
+      and writing blocks in milliseconds.
       The number of blocks shown for an
       upper-level node includes those used by all its child nodes.  In text
       format, only non-zero values are printed.  This parameter may only be