doc: mention statistics reset during crash recovery

Takayuki Tsunakawa

doc/src/sgml/monitoring.sgml

@@ -185,9 +185,11 @@ postgres: <replaceable>user</> <replaceable>database</> <replaceable>host</> <re
    <filename>pg_stat_tmp</filename> by default.
    For better performance, <varname>stats_temp_directory</> can be
    pointed at a RAM-based file system, decreasing physical I/O requirements.
-   When the server shuts down, a permanent copy of the statistics
+   When the server shuts down cleanly, a permanent copy of the statistics
    data is stored in the <filename>global</filename> subdirectory, so that
-   statistics can be retained across server restarts.
+   statistics can be retained across server restarts.  When recovery is
+   performed at server start (e.g. after immediate shutdown, server crash,
+   and point-in-time recovery), all statistics counters are reset.
   </para>
 
  </sect2>