[gtk] docs: Mention G_ENABLE_DIAGNOSTIC in the porting notes

[Emmanuele Bassi <ebassi src gnome org>]

commit 5c3c95a96cf09f3f25605e1d7b8796b644c75ac4
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Fri Feb 9 19:36:00 2018 +0000

    docs: Mention G_ENABLE_DIAGNOSTIC in the porting notes
    
    Since part of our type system is resolved at run time, we need to use
    run time facilities to get diagnostic messages, like deprecation
    warnings for properties and signals.
    
    We should mention the G_ENABLE_DIAGNOSTIC environment variable in the
    migration guide, to let developers know how to catch deprecations and
    changes while porting their code.

 docs/reference/gtk/migrating-3to4.xml | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index ef6fa58e01..46327e43af 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -52,6 +52,21 @@
       </para>
     </section>
 
+    <section>
+      <title>Enable diagnostic warnings</title>
+      <para>
+        Deprecations of properties and signals cannot be caught at compile
+        time, as both properties and signals are installed and used after
+        types have been instantiated. In order to catch deprecations and
+        changes in the run time components, you should use the
+        <envar>G_ENABLE_DIAGNOSTIC</envar> environment variable when
+        running your application, e.g.:
+        <programlisting>
+          G_ENABLE_DIAGNOSTIC=1 ./your-app
+        </programlisting>
+      </para>
+    </section>
+
     <section>
       <title>Do not use widget style properties</title>
       <para>