[css-grid-1] Improve description of 'auto' track sizes. w3c#3571

Author: fantasai

css-grid-1/Overview.bs

@@ -1485,12 +1485,25 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 
 		<dt><dfn>auto</dfn>
 		<dd>
-			As a maximum, identical to ''max-content''.
-			As a minimum, represents the largest minimum size (as specified by 'min-width'/'min-height')
+			As a <em>maximum</em>: represents the largest <a>max-content contribution</a>
+			of the <a>grid items</a> occupying the <a>grid track</a>;
+			however, unlike ''max-content'',
+			allows expansion of the track
+			by the 'align-content' and 'justify-content' properties.
+
+			As a <em>minimum</em>: represents the largest <a>minimum size</a>
+			(specified by 'min-width'/'min-height')
 			of the <a>grid items</a> occupying the <a>grid track</a>.
-
-			Note: ''grid-template-rows/auto'' track sizes (and only ''grid-template-rows/auto'' track sizes)
-			can be stretched by the 'align-content' and 'justify-content' properties.
+			(This initially is often, but not always,
+			equal to a ''min-content'' minimum--
+			see [[#min-size-auto]].)
+
+			When appearing outside a ''minmax()'' notation:
+			equivalent to ''minmax(auto, auto)'',
+			representing the range between
+			the minimum and maximum described above.
+			(This behaves similar to ''minmax(min-content, max-content)'' in the most basic cases,
+			but with extra abilities.)
 
 		<dt><dfn lt="fit-content()">fit-content( <<length-percentage>> )</dfn>
 		<dd>