[css-inline-3] Clarify wording around inline kerning of initial letter boxes.

Author: fantasai

css-inline-3/Overview.bs

@@ -2213,13 +2213,15 @@ Inline Kerning</h4>
 	If the <a>initial letter</a> is a non-atomic inline
 	with an [=automatic size|automatic=] [=inline size=] and
 	zero padding and borders,
-	it is negatively offset
+	its [=margin box=] is kerned (negatively inset)
 	by the distance from the start edge of its content box
 	to the point in the content that would have been placed
-	at the start edge of the containing block
+	at the start edge of the [=line box=]
 	if it were not an <a>initial letter</a>
 	(i.e. the distance between its glyph bounding box
 	and its start side bearing).
+	This inset is effectively
+	an additional [=inline-start=] [=margin=] on the box.
 
 <h3 id="initial-letter-wrapping">
 Initial Letter Wrapping: the 'initial-letter-wrap' property</h3>