[css-grid-2] Clarify that subgrids don't establish independent formatting contexts.

Author: fantasai

css-grid-2/Overview.bs

@@ -104,7 +104,10 @@ Establishing a Subgrid</h3>
 			indicates that the grid will adopt the spanned portion of its parent grid in that axis.
 			Rather than being specified explicitly,
 			the sizes of the grid rows/columns
-			will be taken from the parent grid's definition.
+			will be taken from the parent grid's definition,
+			and the <a>subgrid</a>’s items will participate
+			in the <a href="https://www.w3.org/TR/css-grid-1/#algo-content">intrinsic size calculations</a> ([[css-grid-1#algo-content]])
+			of any tracks shared with the parent grid.
 
 			The <<line-name-list>> argument allows local naming of the grid lines propagated from the parent grid:
 			if a <<line-name-list>> is given,
@@ -117,6 +120,10 @@ Establishing a Subgrid</h3>
 			the initial value, ''grid-template-rows/none''.
 	</dl>
 
+	Unlike those of a regular nested grid,
+	a subgrid’s contents participate in its parent <a>grid formatting contex</a>;
+	thus a subgrid does not establish an <a>independent formatting context</a>.
+
 	The syntax of <<line-name-list>> is defined as follows:
 
 	<pre class="prod">