[gnome-shell] status/volume: Document public function

[Marge Bot <marge-bot src gnome org>]

commit d80a7224cdbe6c0f00747b7795216a317a41a64e
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Jul 15 14:23:23 2022 +0200

    status/volume: Document public function
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2380>

 js/ui/status/volume.js | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/js/ui/status/volume.js b/js/ui/status/volume.js
index 744d650e9b..bbbfb454be 100644
--- a/js/ui/status/volume.js
+++ b/js/ui/status/volume.js
@@ -19,6 +19,9 @@ const VolumeType = {
 // Each Gvc.MixerControl is a connection to PulseAudio,
 // so it's better to make it a singleton
 let _mixerControl;
+/**
+ * @returns {Gvc.MixerControl} - the mixer control singleton
+ */
 function getMixerControl() {
     if (_mixerControl)
         return _mixerControl;