[gtksourceview] docs: update docs for current-line-number

[Christian Hergert <chergert src gnome org>]

commit 64a06166ec6d21a91055a957c1d1bb143164f9a1
Author: Christian Hergert <chergert redhat com>
Date:   Tue Dec 21 23:21:46 2021 -0800

    docs: update docs for current-line-number

 docs/style-reference.md | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/docs/style-reference.md b/docs/style-reference.md
index 8309b295..4ab8a932 100644
--- a/docs/style-reference.md
+++ b/docs/style-reference.md
@@ -141,7 +141,10 @@ textview.
 - `current-line-number`
 
 Current line style for the left margin, on which the current
-line number is drawn. Only the `background` attribute is used.
+line number is drawn. The `background`, `foreground`, and
+`bold` attributes may be used. `foreground` and `bold` were
+added in GtkSourceView 5.4. If `bold` is unset, it defaults
+to `true`.
 
 - `bracket-match`