Docs: add 'volume' argument to fade_in comments

src/aura_engine/core/channels.py

@@ -235,6 +235,7 @@ class GenericChannel:
         Perform a fade-in for the given channel.
 
         Args:
+            volume (int): Volume from -1 to 100, where -1 indicates an error
             instant(bool): If true the fade instantly jumps to target volume
 
         Returns: