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

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">