add a link to up and down mixing in connect(audionode)

This fixes WebAudio#1797.

Author: padenot

index.bs

@@ -2995,7 +2995,7 @@ Methods</h4>
 		{{AudioNode}} object.
 
 		<pre class=argumentdef for="AudioNode/connect(destinationNode, output, input)">
-			destinationNode: The <code>destination</code> parameter is the {{AudioNode}} to connect to. <span class="synchronous">If the <code>destination</code> parameter is an {{AudioNode}} that has been created using another {{AudioContext}}, an {{InvalidAccessError}} MUST be thrown</span>. That is, {{AudioNode}}s cannot be shared between {{AudioContext}}s.
+			destinationNode: The <code>destination</code> parameter is the {{AudioNode}} to connect to. <span class="synchronous">If the <code>destination</code> parameter is an {{AudioNode}} that has been created using another {{AudioContext}}, an {{InvalidAccessError}} MUST be thrown</span>. That is, {{AudioNode}}s cannot be shared between {{AudioContext}}s. Multiple {{AudioNode}} can be connected to the same {{AudioNode}}, this is described in the section [[#channel-up-mixing-and-down-mixing|Channel Upmixing and down mixing]].
 			output: The <code>output</code> parameter is an index describing which output of the {{AudioNode}} from which to connect. <span class="synchronous">If this parameter is out-of-bound, an {{IndexSizeError}} exception MUST be thrown.</span> It is possible to connect an {{AudioNode}} output to more than one input with multiple calls to connect(). Thus, "fan-out" is supported.
 			input:  The <code>input</code> parameter is an index describing which input of the destination {{AudioNode}} to connect to. <span class="synchronous">If this parameter is out-of-bounds, an {{IndexSizeError}} exception MUST be thrown.</span> It is possible to connect an {{AudioNode}} to another {{AudioNode}} which creates a <dfn dfn for>cycle</dfn>: an {{AudioNode}} may connect to another {{AudioNode}}, which in turn connects back to the input or {{AudioParam}} of the first {{AudioNode}}. This is allowed only if there is at least one {{DelayNode}} in the <em>cycle</em> <span class="synchronous">or a {{NotSupportedError}} exception MUST be thrown.</span>
 		</pre>