DOCS-13223 addStorage error codes (#1599)

* DOCS-13223 addStorage error codes

* DOCS-13223 addStorage error codes

Author: Dave Cuthbert

source/reference/exit-codes.txt

@@ -89,6 +89,12 @@ issues with :binary:`~bin.mongod` and :binary:`~bin.mongos` instances.
 
        .. symbol: EXIT_NET_ERROR = 48
 
+   * - .. error:: 61
+     - Returned by the :ref:`storage-node-watchdog` if any of the 
+       filesystems with monitored directories are unresponsive.
+
+       .. symbol: EXIT_WATCHDOG = 61
+
    * - .. error:: 62
      - Returned by :binary:`~bin.mongod` if the datafiles in
        :option:`--dbpath <mongod --dbpath>` are incompatible with the