doc: mention vacuum's removal of commit timestamp information

Reported-by: Kyotaro HORIGUCHI
Discussion: https://postgr.es/m/20180622.172132.230342845[email protected]

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

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 71713f471d72c912cf5e71161b9495111a8f4265..f971c7f0ee3233eba6dd23602b70bfdb0ad89096 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -26006,7 +26006,8 @@ 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">