[css-grid-3] Fix description of row/column keyword. Expand description of normal keyword.

Author: tabatkins

css-grid-3/Overview.bs

@@ -1273,10 +1273,7 @@ Item Flow Axis: 'item-track'/'item-direction'</h3>
 			<span class="option masonry">Track-oriented Option</span>
 			Represents placement into rows,
 			i.e. tracks or lines parallel to the [=inline axis=].
-			Items are placed in start-to-end order in the inline axis
-			in [=flex layout=] and [=grid layout=],
-			and in start-to-end order in the [=block axis=]
-			in [=masonry layout=].
+			Items fill those rows in start-to-end order.
 
 			<span class="option grid">Flow-oriented Option</span>
 			Represents row-primary item placement,
@@ -1289,10 +1286,7 @@ Item Flow Axis: 'item-track'/'item-direction'</h3>
 			<span class="option masonry">Track-oriented Option</span>
 			Represents placement into columns,
 			i.e. tracks or lines parallel to the [=block axis=].
-			Items are placed in start-to-end order in the block axis
-			in [=flex layout=] and [=grid layout=],
-			and in start-to-end order in the [=inline axis=]
-			in [=masonry layout=].
+			Items fill those columns in start-to-end order.
 
 			<span class="option grid">Flow-oriented Option</span>
 			Represents column-primary item placement,
@@ -1350,11 +1344,24 @@ Item Cross Axis Placement Mode: 'item-cross'/'item-wrap'</h3>
 
 		: <dfn>normal</dfn>
 		::
-			Items are placed in start-to-end order in the axis opposite to the [=primary track axis=].
+			Items are placed in start-to-end order
+			in the axis opposite to the [=primary track axis=].
+
+			In [=flex layout=] and [=grid layout=],
+			this controls the direction that new tracks
+			(flex lines or grid tracks)
+			are placed in.
+
+			In [=masonry layout=],
+			<span class="option masonry">for track-oriented syntax this controls which track is selected
+			when several are tied for equal height</span>;
+			<span class="option grid">for flow-oriented syntax this controls which direction
+			items fill their track in.</span>
 
 		: <dfn>reverse</dfn>
 		::
-			Items are placed in end-to-start order in the axis opposite to the [=primary track axis=].
+			Items are placed in end-to-start order
+			in the axis opposite to the [=primary track axis=].
 
 		: <dfn>wrap-reverse</dfn>
 		::
@@ -1565,4 +1572,4 @@ track-sizing/masonry-track-sizing-explicit-block.html
 track-sizing/masonry-track-sizing-overflow-left-side.html
 track-sizing/masonry-track-sizing-overflow-right-side.html
 track-sizing/masonry-track-sizing-span-row.html
-</wpt>
+</wpt>