[css-animations] Clarify CSSKeyframesRule.appendRule(); valid rule is always appended

Author: Sylvain Galineau

css-animations/Overview.bs

@@ -962,7 +962,7 @@ Attributes</h4>
 The <code>appendRule</code> method</h4>
 
 	The <dfn method for="CSSKeyFramesRule">appendRule()</dfn> method appends the passed
-	<a idl>CSSKeyframeRule</a> into the list at the passed key.
+	<a idl>CSSKeyframeRule</a> at the end of the keyframes rule.
 
 	Parameters:
 
@@ -971,7 +971,8 @@ The <code>appendRule</code> method</h4>
 		<dt><dfn argument for="CSSKeyFramesRule/appendRule()">rule</dfn> of type <a interface>DOMString</a>
 		<dd>
 			The rule to be appended, expressed in the same syntax as one entry in the
-			''@keyframes'' rule.
+			''@keyframes'' rule. A valid rule is always appended e.g. even if its key already
+			exists.
 	</dl>
 
 	No Return Value