[css-grid-3][masonry] Simplify alignment description now that align/j…

…ustify-tracks were removed.

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>