[css-align] Apply the fixes to grid too.

Author: tabatkins

css-align/Overview.bs

@@ -678,7 +678,12 @@ Baseline Content-Alignment</h3>
 			A <a>grid item</a> participates in first (last) <a>baseline content-alignment</a>
 			in either its row or column (whichever matches its <a>inline axis</a>)
 			if its computed 'align-content' is ''baseline'' (''last-baseline''),
-			and its computed 'align-self' or 'justify-self' (whichever affects its <a>block axis</a>) is ''justify-self/stretch'' or ''start'' (''end'').
+			and its computed 'align-self' or 'justify-self' (whichever affects its <a>block axis</a>)
+			is ''align-self/stretch'' or ''self-start'' (''self-end'').
+			For this purpose,
+			the ''start'', ''end'', ''flex-start'', and ''flex-end'' values of 'align-self'
+			are treated as either ''self-start'' or ''self-end'',
+			whichever they end up equivalent to.
 	</dl>
 
 	If a box spans multiple <a>shared alignment contexts</a>,