[gtksourceview] style-reference: add note about when weight was introduced

[Christian Hergert <chergert src gnome org>]

commit def746ad72409d40d5f7b74c46f30c6b561d0ce5
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jan 17 14:03:37 2022 -0800

    style-reference: add note about when weight was introduced

 docs/style-reference.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/docs/style-reference.md b/docs/style-reference.md
index e3cce236..643c94bf 100644
--- a/docs/style-reference.md
+++ b/docs/style-reference.md
@@ -85,6 +85,8 @@ Background color.
 
 - `weight`
 
+The weight attribute was added in GtkSourceView 5.4.
+
 The weight of the matched text block. The default value is "normal" but
 you may use any of the [enum@Pango.Weight] nicks or optionally a numeric
 value equivalent to the weights of CSS.