Update example for NodeStore backends

rippled-example.cfg

@@ -223,9 +223,25 @@
 #              shfArahZT9Q9ckTf3s1psJ7C7qzVN
 #
 # [node_db]
-#   Sets the database back end used for hashed nodes and the parameters
-#   for that back end, separated by pipe characters. For LevelDB,
-#   use "type=LevelDB|Path=db/hashnode" for MDB, use "type=mdb|path=db".
+# [temp_db]
+#   Set the choice of databases for storing Node objects.
+#   Format (without spaces):
+#       <key> '=' <value> [ '|' <key> '=' <value> ]...
+#   Examples:
+#       type=HyperLevelDB|path=db/hashnode
+#   Choices for 'type':
+#       HyperLevelDB        Use an improved version of LevelDB (preferred)
+#       LevelDB             Use Google's LevelDB database (deprecated)
+#       MDB                 Use MDB
+#       KeyvaDB             Use OpenCoin's KeyvaDB (experimental)
+#   Required keys:
+#       path                Location to store the database (all types)
+#   Optional keys:
+#       ...
+#   Notes
+#       The 'node_db' entry configures the primary, persistent storage.
+#       The 'temp_db' configures a look-aside cache for high volume storage
+#           which doesn't necessarily persist between server launches.
 #
 # [node_size]
 #   Tunes the servers based on the expected load and available memory. Legal