Update AudioAnalyzer.yaml (#911)

## Changes

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

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---------

Co-authored-by: Andrew Etter <[email protected]>

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.
+      If any of the analyzer's inputs come from an `Class.AudioDeviceInput`, this method returns an empty array.
     code_samples: []
     parameters: []
     returns: