Updated Audio management (markdown)

Audio-management.md

@@ -4,6 +4,8 @@ Audio management in SDRangel is divided into two parts
 
 From the main window top menu open "Preferences" then "Audio". Then check this file: https://github.com/f4exb/sdrangel/blob/master/sdrgui/audio.md
 
+:warning: **You may not change the audio device settings of an audio device that is in use.**
+
 <h2>Audio device selection</h2>
 
 The device used for audio can be selected for each channel plugin having audio. The control depends on the plugin.