Fix #2309: OscillatorNode output is mono (#2310)

Add a sentence (like we have for `ConstantSourceNode`) that says the
`OscillatorNode` has a single output that is mono.

Not exactly sure where the best place is to put this sentence, though.

Co-authored-by: Raymond Toy <rtoy@chromium.org>

index.bs

@@ -8276,6 +8276,8 @@ time integral of <a>computedOscFrequency</a>, assuming a phase angle
 of zero at the node's exact start time. Its <a>nominal range</a> is
 [-<a>Nyquist frequency</a>, <a>Nyquist frequency</a>].
 
+The single output of this node consists of one channel (mono).
+
 <pre class=include>
 path: audionode.include
 macros: