[css-align] Fix errors in flex baseline content-alignment section

Author: tabatkins

css-align/Overview.bs

@@ -665,8 +665,13 @@ Baseline Content-Alignment</h3>
 		<dd>
 			A <a>flex item</a> participates in first (last) <a>baseline content-alignment</a>
 			in its flex line if
-			its computed 'align-content' is ''baseline'' (''last-baseline'')
-			and its computed 'align-self' is ''align-self/stretch'' or ''start'' (''end'').
+			its computed 'align-content' is ''baseline'' (''last-baseline''),
+			its <a>inline axis</a> is parallel to the <a>main axis</a>,
+			and its computed 'align-self' 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 ''start'' or ''end'',
+			whichever they end up equivalent to.
 
 		<dt>Grid Items:
 		<dd>