[pango/pango2: 4/5] docs: Update migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit 620a6b51e7716d2098e49e47ba291297ac3326c4
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jun 16 07:26:17 2022 -0400

    docs: Update migration guide

 docs/migrating.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/docs/migrating.md b/docs/migrating.md
index 05f50ce1..3cd97166 100644
--- a/docs/migrating.md
+++ b/docs/migrating.md
@@ -48,6 +48,11 @@ meson option. But if it is enabled, it no longer requires linking against
 a separate shared library or using a separate pkgconfig file. Just include
 the `pangocairo.h` header, and use the APIs that are declared in it.
 
+There is still a `pangocairo2.pc` file, if you want to be explicit in your
+build configuration about requiring cairo support. If you want to handle
+the possible absence of cairo support at runtime, you can check the
+`PANGO_CAIRO_RENDERING` macro before including `pangocairo.h`.
+
 ### PangoFontMap changes
 
 The `PangoFontMap` class has seen some significant changes. It is now possible