[DOCS] Adds data_counts object to the GET DFA stats API (#54498)

docs/reference/ml/ml-shared.asciidoc

@@ -867,6 +867,33 @@ include::{docdir}/ml/ml-shared.asciidoc[tag=dfas-validation-loss-fold]
 For running jobs only, contains messages relating to the selection of a node to 
 run the job.
 
+//Begin data_counts
+`data_counts`:::
+(object)
+An object containing statistical data about the documents in the analysis.
++
+.Properties of `data_counts`
+[%collapsible%open]
+====
+`skipped_docs_count`:::
+(integer)
+The number of documents that are skipped during the analysis because they 
+contained values that are not supported by the analysis. For example, 
+{oldetection} does not support missing fields so it skips documents with missing 
+fields. Likewise, all types of analysis skip documents that contain arrays with 
+more than one element.
+
+`test_docs_count`:::
+(integer)
+The number of documents that are not used for training the model and can be used 
+for testing.
+
+`training_docs_count`:::
+(integer)
+The number of documents that are used for training the model.
+====
+//End data_counts
+
 `id`:::
 (string)
 The unique identifier of the {dfanalytics-job}.