doc: mention vacuum's removal of commit timestamp information

bmomjian · bmomjian · commit c136eb029815 · 2023-11-24T17:56:28.000-05:00
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
@@ -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">
