[css-sizing] Fix definition of percentage sizes.

Author: fantasai

css-sizing/Overview.bs

@@ -563,16 +563,20 @@ Fill-available Sizing</h3>
 <h3 id="percentage-sizing">
 	Percentage Sizing</h3>
 
-	Issue: Write something useful here. Like about when to fall back to auto or whatever.
-
-	In order to prevent cyclic sizing,
-	the ''min-height/auto'' value of 'min-height' and 'max-height'
-	does not factor into the percentage size resolution of the box’s contents.
-	For example, a percentage-height block
-	whose <a>flex item</a> parent has
-	''height: 120em; min-height: auto'' will size itself against ''height: 120em''
-	regardless of the impact that 'min-height' might have
-	on the used size of the <a>flex item</a>.
+	Percentages specify sizing of a box with respect to the box’s <a>containing block</a>.
+
+	Although this may require an additional layout pass to re-resolve percentages in some cases,
+	the ''min-width/auto'', ''min-width/min-content'', ''min-width/max-content'', and ''min-width/fit-content'' values
+	of 'min-width' and 'min-height'
+	do not prevent the resolution of percentage sizes of the box’s contents.
+	However, in order to prevent cyclic sizing in the general case,
+	percentages do not otherwise resolve against <a>indefinite</a> sizes,
+	and instead are treated as <css>auto</css>.
+
+	Note: See definition of percentage
+	<a href="https://www.w3.org/TR/CSS2/visudet.html#the-width-property"><css>width</css></a>
+	and <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property"><css>height</css></a>
+	in [[CSS21]].
 
 <h2 class=no-num id="changes">
 Changes</h2>