Update AudioAnalyzer.yaml

Add a note about AudioAnalyzer:GetSpectrum returning an empty array.

Author: rbxphogen

content/en-us/reference/engine/classes/AudioAnalyzer.yaml

@@ -117,6 +117,7 @@ methods:
       Returns the frequency spectrum of the last audio buffer, as an array of
       numbers. The elements of the array are root-mean-square volume levels,
       evenly spaced from 0 hertz to 24,000 hertz.
+      However, if any of the analyzer's inputs come from an `Class.AudioDeviceInput`, this method returns an empty array.
     code_samples: []
     parameters: []
     returns: