[gtk/matthiasc/for-master: 1/2] docs: Document GTK_MEDIA

[Matthias Clasen <matthiasc src gnome org>]

commit 6b5dff5667ddfa9c493fe2d6b09e7750f523cb24
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Nov 8 09:58:26 2020 -0500

    docs: Document GTK_MEDIA
    
    Add a paragraph about the GTK_MEDIA environment variable.

 docs/reference/gtk/running.md | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/docs/reference/gtk/running.md b/docs/reference/gtk/running.md
index 2135eaa8d5..d0da90a5df 100644
--- a/docs/reference/gtk/running.md
+++ b/docs/reference/gtk/running.md
@@ -98,6 +98,14 @@ be used for the default IM module. This also can be a colon-separated
 list of input-methods, which GTK will try in turn until it finds one
 available on the system.
 
+### GTK_MEDIA
+
+Specifies what backend to load for #GtkMediaFile. The possible values
+depend on what options GTK was built with, and can include 'gstreamer',
+'ffmpeg' and 'none'. If set to 'none', media playback will be unavailable.
+The special value 'help' can be used to obtain a list of all supported
+media backends.
+
 ### GTK_EXE_PREFIX
 
 If set, GTK uses `$GTK_EXE_PREFIX/lib` instead of the libdir