[css-sizing] Add note about what fill means as requested in #317.

Author: fantasai

css-sizing-3/Overview.bs

@@ -258,6 +258,11 @@ New Keywords for 'width' and 'height'</h3>
 			Use the <a>fill-available inline size</a> or <a>fill-available block size</a>,
 			as appropriate to the writing mode.
 
+			NOTE: This is the formula used to calculate ''width/auto'' widths
+			for non-replaced blocks in normal flow, see <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
+			It allows re-using this formula for boxes that are otherwise shrink-wrapped,
+			like tables.
+
 		<dt><dfn>max-content</dfn>
 		<dd>
 			Use the <a>max-content inline size</a> or <a>max-content block size</a>,

css-sizing/Overview.bs

@@ -259,6 +259,11 @@ New Keywords for 'width' and 'height'</h3>
 			Use the <a>fill-available inline size</a> or <a>fill-available block size</a>,
 			as appropriate to the writing mode.
 
+			NOTE: This is the formula used to calculate ''width/auto'' widths
+			for non-replaced blocks in normal flow, see <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
+			It allows re-using this formula for boxes that are otherwise shrink-wrapped,
+			like tables.
+
 		<dt><dfn>max-content</dfn>
 		<dd>
 			Use the <a>max-content inline size</a> or <a>max-content block size</a>,