[css-align] tighten the language around when flex-start/end computes to start/end.

Author: tabatkins

css-align/Overview.bs

@@ -225,14 +225,14 @@ Positional Alignment: the ''center'', ''start'', ''end'', ''self-start'', ''self
 		<dd><strong>Only used in flex layout.</strong> [[!CSS3-FLEXBOX]]
 			Aligns the <a>alignment subject</a> to be flush with the edge of the <a>alignment container</a>
 			corresponding to the <a>flex container</a>’s main-start or cross-start side, as appropriate.
-			When used in layout modes other than Flexbox,
+			When used on boxes that are not children of a <a>flex container</a>,
 			this value computes to ''start''.
 
 		<dt><dfn for="<self-position>, <content-position>">flex-end</dfn> <small>(self, content)</small>
 		<dd><strong>Only used in flex layout.</strong>
 			Aligns the <a>alignment subject</a> to be flush with the edge of the <a>alignment container</a>
 			corresponding to the <a>flex container</a>’s main-end or cross-end side, as appropriate.
-			When used in layout modes other than Flexbox,
+			When used on boxes that are not children of a <a>flex container</a>,
 			this value computes to ''end''.
 
 		<dt><dfn for="<self-position>, <content-position>">left</dfn> <small>(self, content)</small>