fix: mention dataset families in web client

docs/web_client.md

@@ -30,6 +30,9 @@ You should land on the following page.
 You can then select the dataset of your choice to search into.
 For an example, try the **VIPERS-W1 (DR2)** dataset.
 
+**Note**: datasets are grouped into families. Families of datasets can be defined as related datasets of common interest.
+For instance, on the previous picture the *Default dataset family*.
+
 ### Adding criteria
 
 A form should now appear in front of you.