[pango/pango2] Migration guide: updates

[Matthias Clasen <matthiasc src gnome org>]

commit 1df975dea3dd592882a4b078f962f8dce912303f
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jun 16 22:42:53 2022 -0400

    Migration guide: updates

 docs/migrating.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/docs/migrating.md b/docs/migrating.md
index 3cd97166..339b5384 100644
--- a/docs/migrating.md
+++ b/docs/migrating.md
@@ -93,6 +93,15 @@ and strikethroughs, and a separate [enum@Pango.UnderlinePosition] enumeration.
 Alpha attributes have been removed, since [struct@Pango.Color] now contains
 an alpha field.
 
+### Markup changes
+
+The attributes 'alpha', 'fgalpha', 'background_alpha' and 'bgalpha' have
+been removed. Alpha can now be specified as part of color attributes.
+
+The 'underline', 'overline' and 'strikethrough' attributes now all take values
+from the [enum@Pango.LineStyle] enumeration. The `PANGO_UNDERLINE_LOW` value
+has been replaced by the separate 'underline-position' attribute.
+
 ## New functionality to explore
 
 ### Complex line breaking with PangoLineBreaker