Add note about switching accounting_storage plugins at the same time

doing an upgrade.

It isn't advised.  Do one then the other.  Basically if you are using the
mysql plugin make sure you add the cluster to the system as the mysql
plugin doesn't do that explicitly.

Bug 3131

doc/html/accounting.shtml

@@ -81,6 +81,22 @@ is to execute.  If you want munge authentication, which is highly
 recommended, you will also need to install
 the <i>slurm-munge</i> RPM.</p>
 
+<p>Note if you switch from using the MySQL plugin to use the SlurmDBD plugin
+  you must make sure the cluster has been added to the database.  The MySQL
+  plugin doesn't require this, but also will not hurt things if you have it
+  there when using the MySQL plugin.  You can verify with
+<pre>
+sacctmgr list cluster
+</pre>
+If the cluster isn't there add it (where my clusters name were snowflake):
+<pre>
+sacctmgr add cluster snowflake
+</pre>
+Failure to do so will result in the slurmctld failing to talk to the slurmdbd
+after the switch.  If you plan to upgrade to a new version of Slurm don't switch
+plugins at the same time or you may get unexpected results.  Do one then the
+other.</p>
+
 <p>If SlurmDBD is configured for use but not responding then <i>slurmctld</i> 
 will utilize an interal cache until SlurmDBD is returned to service.
 The cached data is written by <i>slurmctld</i> to local storage upon shutdown