[css-overflow-3] Clarify compatibilty value and property

The -webkit- prefixed compat part of line-clamp and related properties
aren't meant to be anything out of ordinary in terms of property
mechanics, cascading, OM, etc. They're defined in a separate section do
deemphasize them, but otherwise behave normally. The previous
description was confusing about that, so this clarifies that.

See #6842

Author: frivoal

css-overflow-3/Overview.bs

@@ -1398,8 +1398,22 @@ Limiting Visible Lines: the 'line-clamp' shorthand property</h3>
 Legacy compatibility</h4>
 
 	For compatibility with legacy content,
-	UAs that support 'line-clamp' must also support the <dfn dfn-type=property>-webkit-line-clamp</dfn> property.
-	Like 'line-clamp', it is a shorthand of 'max-lines', 'continue', and 'block-ellipsis',
+	UAs that support 'line-clamp' must also support the '-webkit-line-clamp' property
+	and the additional ''-webkit-discard'' value for the 'continue' property.
+
+	<pre class="propdef shorthand">
+		Name: -webkit-line-clamp
+		Value: none | <<integer>>
+		Initial: none
+		Percentages: N/A
+	</pre>
+
+	<pre class="propdef partial">
+		Name: continue
+		New Values: -webkit-discard
+	</pre>
+
+	Like 'line-clamp', '-webkit-line-clamp' is a shorthand of 'max-lines', 'continue', and 'block-ellipsis',
 	except that:
 
 	* its syntax is ''line-clamp/none'' | <<integer>>