Document [path_search_size] in rippled-example.cfg.

rippled-example.cfg

@@ -235,6 +235,12 @@
 # [database_path]:
 #   Full path of database directory.
 #
+# [path_search_size]
+#   When searching for paths, the maximum number of nodes allowed. This can take
+#   exponentially more resource as the size is increasded.
+#
+#   The default is: 5
+#
 # [rpc_startup]:
 #   Specify a list of RPC commands to run at startup.
 #