Update tutorial to discuss empty clusters and LloydStepType.

doc/tutorials/kmeans/kmeans.txt

@@ -653,11 +653,13 @@ The \c LloydStepType policy also mandates three functions:
 @code
 /**
  * Run a single iteration of the Lloyd algorithm, updating the given centroids
- * into the newCentroids matrix.
+ * into the newCentroids matrix.  If any cluster is empty (that is, if any
+ * cluster has no points assigned to it), then the centroid associated with
+ * that cluster may be filled with invalid data (it will be corrected later).
  *
  * @param centroids Current cluster centroids.
  * @param newCentroids New cluster centroids.
- * @param counts Counts of the number of points in each cluster.
+ * @param counts Number of points in each cluster at the end of the iteration.
  */
 double Iterate(const arma::mat& centroids,
                arma::mat& newCentroids,
@@ -670,6 +672,10 @@ double Iterate(const arma::mat& centroids,
 size_t DistanceCalculations() const { return distanceCalculations; }
 @endcode
 
+Note that \c Iterate() does not need to return valid centroids if the cluster is
+empty.  This is because \c EmptyClusterPolicy will handle the empty centroid.
+This behavior can be used to avoid small amounts of computation.
+
 For examples, see the five aforementioned implementations of classes that
 satisfy the \c LloydStepType policy.