Update metrics.rst -- say what bottom row of classification_report_imbalanced output contains

Especially point out that the Weighted Average Recall that `classification_report_imbalanced` displays is also better known as the Accuracy!  Maybe this should also be a change in the API docs for  `classification_report_imbalanced`  but the "Edit" link there didn't work.

Author: DavidRosen

doc/metrics.rst

@@ -68,7 +68,12 @@ Summary of important metrics
 The :func:`classification_report_imbalanced` will compute a set of metrics
 per class and summarize it in a table. The parameter `output_dict` allows
 to get a string or a Python dictionary. This dictionary can be reused to create
-a Pandas dataframe for instance.
+a Pandas dataframe for instance.  Its "avg/total" bottom row contains 
+the Weighted Average of each column (weighting is by the sup (Support), i.e. by
+the true class proportions), except the sup (Support) column whose total
+appears at its bottom.  Also note that the avg value at the bottom of the "rec"
+column is simply the Accuracy because  Weighted Average Recall is just another 
+name for Accuracy.
 
 .. _pairwise_metrics: