add more emphasis to sound_system config note regarding GMC

Author: bosh

docs/config/sound_system.md

@@ -9,7 +9,7 @@ title: "sound_system:"
 
 !!! warning "Deprecated in MPF 0.80
 
-    The GMC used by MPF 0.80 manages audio outside of configuration files. See the [GMC Setup Guide](../gmc/setup.md) for instructions on configuring the audio system in GMC.
+    The GMC used by MPF 0.80 manages audio outside of YAML configuration files. See the [GMC Setup Guide](../gmc/setup.md) for instructions on configuring the audio system in GMC. The `sound_system:` configuration should be removed for projects upgrading to MPF 0.80 and Godot.
 
 
 | Valid in | |