[css-syntax-3] Add note about how rule contents actually get parsed (it's via grammar).

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2477,6 +2477,18 @@ Consume a simple block</h4>
 			and append it to the value of the block.
 	</dl>
 
+	Note: CSS has an unfortunate syntactic ambiguity
+	between blocks that can contain declarations
+	and blocks that can contain qualified rules,
+	so any "consume" algorithms that handle rules
+	will initially use this more generic algorithm
+	rather than the more specific
+	[=consume a list of declarations=]
+	or [=consume a list of rules=] algorithms.
+	These more specific algorithms are instead invoked when grammars are applied,
+	depending on whether it contains a <<declaration-list>>
+	or a <<rule-list>>/<<stylesheet>>.
+
 
 <h4 id="consume-function">
 Consume a function</h4>