[css-grid-3] Rename the masonry-fill keywords to be more unique. Also lightly rewrite the text for clarity.

Author: tabatkins

css-grid-3/Overview.bs

@@ -516,27 +516,29 @@ Masonry Cross Direction: the 'masonry-fill' property</h4>
 
 	<pre class=propdef>
 	Name: masonry-fill
-	Value: normal | reverse
+	Value: tie-start | tie-end
 	Initial: normal
 	Inherited: no
 	Applies to: [=masonry containers=]
 	Computed value: as specified
 	Animation type: discrete
 	</pre>
 
-	'masonry-fill' determines what direction auto-placed items are laid out in
-	when multiple tracks are tied for “shortest”.
+	Auto-placed [=masonry items=] are always placed 
+	in whichever track is least filled at that moment.
+	When there are ties for "least filled",
+	'masonry-fill' determines which track to choose.
 
 	<dl dfn-for=masonry-fill dfn-type=value>
-		: <dfn>normal</dfn>
+		: <dfn>tie-start</dfn>
 		::
 			Ties are broken
-			by filling the startmost track
+			by putting the item in the start-most tied position.
 
-		: <dfn>reverse</dfn>
+		: <dfn>tie-end</dfn>
 		::
 			Ties are broken
-			by filling the endmost track.
+			by putting the item in the end-most tied position.
 	</dl>
 
 <h4 id=masonry-flow>