Add description of pg_stat directory into doc.

Back-patch to 9.3 where pg_stat directory was introduced.

doc/src/sgml/monitoring.sgml

@@ -186,7 +186,7 @@ postgres: <replaceable>user</> <replaceable>database</> <replaceable>host</> <re
    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 cleanly, a permanent copy of the statistics
-   data is stored in the <filename>global</filename> subdirectory, so that
+   data is stored in the <filename>pg_stat</filename> subdirectory, so that
    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.

doc/src/sgml/storage.sgml

@@ -103,6 +103,12 @@ Item
  <entry>Subdirectory containing exported snapshots</entry>
 </row>
 
+<row>
+ <entry><filename>pg_stat</></entry>
+ <entry>Subdirectory containing permanent files for the statistics
+  subsystem</entry>
+</row>
+
 <row>
  <entry><filename>pg_stat_tmp</></entry>
  <entry>Subdirectory containing temporary files for the statistics