diff --git a/etc/NEWS b/etc/NEWS
index 0578da899bb..882d97ec423 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -598,8 +598,11 @@ It allows tweaking the thresholds for rename and copy detection.
 
 ---
 *** New user option 'diff-refine-nonmodified'.
-Makes 'diff-refine' highlight added and removed whole lines with the
-same faces as the words added and removed within modified lines.
+When this is non-nil, 'diff-refine' will highlight lines that were added
+or removed in their entirety (as opposed to modified lines, where some
+parts of the line were modified), using the same faces as for
+highlighting the words added and removed within modified lines.  The
+default value is nil.
 
 +++
 *** 'diff-ignore-whitespace-hunk' can now be applied to all hunks.
diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index f914cc76790..14a401667e9 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2283,8 +2283,14 @@ Return new point, if it was moved."
     (overlay-put ol 'face face)))
 
 (defcustom diff-refine-nonmodified nil
-  "If non-nil also highlight as \"refined\" the added/removed lines.
-This is currently only implemented for `unified' diffs."
+  "If non-nil, also highlight the added/removed lines as \"refined\".
+The lines highlighted when this is non-nil are those that were
+added or removed in their entirety, as opposed to lines some
+parts of which were modified.  The added lines are highlighted
+using the `diff-refine-added' face, while the removed lines are
+highlighted using the `diff-refine-removed' face.
+This is currently implemented only for diff formats supported
+by `diff-refine-hunk'."
   :version "30.1"
   :type 'boolean)