[css-overflow-3] Add more description to definition of ink overflow. I don't think we should fully define it here; I think individual effects should say what kind of overflow they create if it's not obvious.

Author: fantasai

css-overflow-3/Overview.bs

@@ -92,7 +92,7 @@ Types of overflow</h2>
 	''transform-style: preserve-3d'' must also be examined.
 
 	There are three different types of overflow,
-	which are handled differently by the CSS UA:
+	which are used for different purposes by the UA:
 
 	<ul>
 		<li><a>ink overflow</a>
@@ -106,15 +106,23 @@ Types of overflow</h2>
 	is the part of that box and its contents that
 	creates a visual effect outside of
 	the box's border box.
+	Ink overflow is the overflow of painting effects defined to not affect layout
+	or otherwise extend the <a>scrollable overflow region</a>,
+	such as <a href="http://www.w3.org/TR/css-backgrounds/#box-shadow">box shadows</a>,
+	<a href="">border images</a>,
+	<a href="https://www.w3.org/TR/css-text-decor-3/">text decoration</a>,
+	overhanging glyphs (with negative side bearings,
+	or with ascenders/descenders extending outside the em box),
+	<a href="https://www.w3.org/TR/CSS2/ui.html#dynamic-outlines">outlines</a>,
+	etc.
+
+	Issue: What about 'hanging-punctuation'?
 
 	Since some effects in CSS (for example, the blurs in
 	'text-shadow' [[CSS3TEXT]] and 'box-shadow' [[CSS3BG]])
 	do not define what visual extent they cover, the extent
 	of the <a>ink overflow</a> is undefined.
 
-	<p class="issue">
-		Should we try to define it at all and just leave pieces undefined?
-
 	The <dfn>ink overflow region</dfn> is the non-rectangular area
 	occupied by the <a>ink overflow</a>, and the
 	<dfn>ink overflow rectangle</dfn> is