doc: mention vacuum's removal of commit timestamp information

Reported-by: Kyotaro HORIGUCHI
Discussion: https://postgr.es/m/20180622.172132.230342845.horiguchi.kyotaro@lab.ntt.co.jp

Co-authored-by: Kyotaro HORIGUCHI
Backpatch-through: 16

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 13ce7f856a35dea5dded476099411fc114b6d566..260a1c91733f3ef2c554f57baeea4b490d09c71d 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -25704,7 +25704,8 @@ SELECT collation for ('foo' COLLATE "de_DE");
     They only provide useful data when the
     <xref linkend="guc-track-commit-timestamp"/> configuration option is
     enabled, and only for transactions that were committed after it was
-    enabled.
+    enabled.  Commit timestamp information is routinely removed during
+    vacuum.
    </para>
 
    <table id="functions-commit-timestamp">