[gtksourceview] docs: update porting guide

[Christian Hergert <chergert src gnome org>]

commit 038954a6a5da7deaefe3aa4868ffff9b78c1c62e
Author: Christian Hergert <chergert redhat com>
Date:   Tue Jul 20 16:05:25 2021 -0700

    docs: update porting guide

 docs/reference/porting-guide-4-to-5.xml | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/docs/reference/porting-guide-4-to-5.xml b/docs/reference/porting-guide-4-to-5.xml
index 5c200ef2..689e7ae9 100644
--- a/docs/reference/porting-guide-4-to-5.xml
+++ b/docs/reference/porting-guide-4-to-5.xml
@@ -89,4 +89,12 @@
     </para>
   </refsect1>
 
+  <refsect1>
+    <title>Style Schemes</title>
+    <para>Since 5.2, GtkSourceView will synthesize an appropriate background color
+    for current-line-number if current-line-number’s background is not specified.
+    Style schemes which have not already set current-line-number should be updated
+    and use "#rgba(0,0,0,0)" to match the previous behavior.</para>
+  </refsect1>
+
 </chapter>