[gtk/ebassi/for-master] docs: Add note on command line arguments

[Emmanuele Bassi <ebassi src gnome org>]

commit 790733673f186e73618074cd11fc08bc32c0524c
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Sep 10 17:53:50 2020 +0100

    docs: Add note on command line arguments
    
    We don't parse them any more, and developers should stop using them when
    migrating from GTK3 to GTK4.

 docs/reference/gtk/migrating-3to4.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 27e6ead12c..f02ae68508 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -49,6 +49,15 @@ application, e.g.:
 G_ENABLE_DIAGNOSTIC=1 ./your-app
 ```
 
+### Do not use GTK-specific command line arguments
+
+GTK4 does not parse command line arguments any more. If you are using
+command line arguments like `--gtk-debug` you should use the `GTK_DEBUG`
+environment variable instead. If you are using `--g-fatal-warnings` for
+debugging purposes, you should use the `G_DEBUG` environment variable, as
+specified by the
+[GLib documentation](https://developer.gnome.org/glib/stable/glib-running.html).
+
 ### Do not use widget style properties
 
 Style properties do not exist in GTK 4. You should stop using them in