Updates FailedToCommitClusterStateException javadoc

Update the FailedToCommitClusterStateException javadoc to specify that
it can only be thrown by master nodes.

Author: joshua-adams-1

server/src/main/java/org/elasticsearch/cluster/coordination/FailedToCommitClusterStateException.java

@@ -17,6 +17,7 @@
 
 /**
  * Exception indicating a cluster state update was published and may or may not have been committed.
+ * This exception can only be thrown by master nodes.
  * <p>
  * If this exception is thrown, then the cluster state update was published, but is not guaranteed
  * to be committed, including the next master node. This exception should only be thrown when there is