[css-syntax-3][editorial] Add another explanatory note to the unicode-range tokenizer.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -1739,6 +1739,14 @@ Consume a unicode-range token</h4>
 	Ensure that the stream [=would start a unicode-range=]
 	before calling this algorithm.
 
+	Note: This token is not produced by the tokenizer under normal circumstances.
+	This algorithm is only called
+	during [=consume the value of a unicode-range descriptor=],
+	which itself is only called as a special case
+	for parsing the '@font-face/unicode-range' descriptor;
+	this single invocation in the entire language
+	is due to a bad syntax design in early CSS.
+
 	1. Consume the [=next input code point|next two input code points=] and discard them.
 
 	2. Consume as many [=hex digits=] as possible,