Improve documentation on new exported columns (#1639)

---------

Co-authored-by: Jonathan de Bruin <jonathandebruinos@gmail.com>

docs/source/progress.rst

@@ -126,17 +126,18 @@ The following variables will be added to your tabular dataset:
   not seen during the screening process.
 - The column titled **asreview_ranking** contains an identifier to
   preserve the rank ordering as described below.
-- The column **ASReview_prior** contains a label ``1`` if a record has been used
-  to train the first iteration of the model.
+- The column **ASReview_prior** contains a label ``1`` if a record has been
+  used to train the first iteration of the model, a label ``0`` if not used
+  for training, and empty when the record was not seen.
 - The column **asreview_label_to_validate** is added in the exploration mode and
   contains the labels initially present in the data.
 - The column **Notes** contain any notes you made during screening.
 
 For RIS files, the
 labels **ASReview_relevant**, **ASReview_irrelevant**, **ASReview_not_seen**,
-and **ASReview_prior** are stored with the `N1` (Notes) tag. In citation
-managers. like Zotero and Endnote, the labels can be used for making
-selections.
+and **ASReview_prior**, **ASReview_validate_relevant/irrelevant/not_seen**
+are stored with the `N1` (Notes) tag. In citation managers like Zotero and
+Endnote, the labels can be used for making selections.
 
 The file is ordered as follows: