Oops, forgot to bump catversion in the Serializable Snapshot Isolation patch.

I thought we didn't need that, but then I remembered that it added a new
SLRU subdirectory, pg_serial. While we're at it, document what pg_serial is.

doc/src/sgml/storage.sgml

@@ -82,6 +82,11 @@ Item
  <entry>Subdirectory containing LISTEN/NOTIFY status data</entry>
 </row>
 
+<row>
+ <entry><filename>pg_subtrans</></entry>
+ <entry>Subdirectory containing information about committed serializable transactions</entry>
+</row>
+
 <row>
  <entry><filename>pg_stat_tmp</></entry>
  <entry>Subdirectory containing temporary files for the statistics

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201102011
+#define CATALOG_VERSION_NO	201102081
 
 #endif