[css-grid-3][masonry] Simplify alignment description now that align/justify-tracks were removed.

Author: fantasai

css-grid-3/Overview.bs

@@ -485,9 +485,8 @@ Alignment and Spacing</h2>
 	[[css-align-3#content-distribution|content-distribution]] is applied
 	to the content as a whole, similarly to how it behaves in block containers.
 	More specifically, the <a>alignment subject</a> is the <dfn>masonry box</dfn>,
-	which is the area between the [=content edge=] of the [=grid container=]
-	and the [=end=] [=margin edge=] of the item that is the furthest away in the [=masonry axis=],
-	as indicated by the dashed border here:
+	which is the smallest rectangle bounding
+	the [=margin boxes=] of all the [=grid items=].
 
 	<figure>
 		<img src="images/masonry-box.png">
@@ -497,22 +496,19 @@ Alignment and Spacing</h2>
 		</figcaption>
 	</figure>
 
-	Note that there is only ever one <a>alignment subject</a>
+	Note: There is only ever one <a>alignment subject</a>
 	for these properties in the [=masonry axis=],
 	so the unique 'align-content' / 'justify-content' values boil down to
 	''align-content/start'',
 	''align-content/center'',
 	''align-content/end'',
-	''align-content/stretch''
 	and [=baseline alignment=].
-	(''align-content/normal'' behaves as ''align-content/stretch'' as usual for [=grid containers=].)
-	In the figure above, the [=grid container=] has ''align-content: start'';
-	but if ''align-content'' were at its default ''align-content/normal'' value,
-	then the [=masonry box=] would fill the [=grid container=]'s content box,
-	due to being stretched.
-	Moreover: if the grid items overflowed
+	(The behavior of ''align-content/normal'' and ''align-content/stretch''
+	is identical to ''align-content/start'',
+	and the [=distributed alignment=] values behave as their [=fallback alignments=].)
+	If the [=grid items=] overflow
 	the [=grid container=]'s [=content box=] in the [=masonry axis=],
-	then the [=masonry box=] would be larger than the [=grid container=]'s [=content box=].
+	then the [=masonry box=] will be larger than the [=grid container=]'s [=content box=].
 
 <h3 id="masonry-axis-baseline-alignment">
 Baseline Alignment in the Masonry Axis</h3>