[css-grid-2] Add some notes pointing out the implications of subgrid clamping. w3c#6863

Author: tabatkins

css-grid-2/Overview.bs

@@ -3554,6 +3554,15 @@ Subgrids</h2>
 					floored at one.
 			</ul>
 
+			Note: The explicit grid determined here can be further truncated
+			if the placement of the [=subgrid=] is clamped by its parent grid.
+			See <a href="#subgrid-implicit">the "no implicit tracks" bullet point</a>.
+
+			If the 'grid-template-*' properties specify a <<line-name-list>>
+			in a [=subgridded axis=],
+			the used value is truncated
+			to match the used number of explicit tracks.
+
 		<li id="subgrid-indexing">
 			The <a>grid-placement properties</a> of the <a>subgrid</a>’s <a>grid items</a>
 			and the line numbers they use
@@ -3659,6 +3668,12 @@ Subgrids</h2>
 				then that item is instead forced into (and limited to) the first (only) track in the subgrid.
 			</div>
 
+			Note: This means that a subgrid might have fewer tracks than it expected,
+			if its parent is also a subgrid and therefore has a fixed number of tracks.
+			(A subgrid might likewise have fewer tracks than expected
+			because its parent is hitting the
+			[[#overlarge-grids|UA limit on grid tracks]].)
+
 		<li id="subgrid-size-contribution">
 			The <a>subgrid</a> itself lays out as an ordinary <a>grid item</a> in its [=parent grid=],
 			but acts as if it was completely empty for track sizing purposes