[css-align-3] Properly mark left/right as being only for the justify-* properties, not for either of the <<*-position>> productions. Fixes w3c#2508.

Author: tabatkins

css-align-3/Overview.bs

@@ -321,15 +321,15 @@ Positional Alignment: the ''center'', ''start'', ''end'', ''self-start'', ''self
 			and on boxes that are not <a>flex containers</a>,
 			this value behaves as ''end'' when used in the <a>content-distribution properties</a>.
 
-		<dt><dfn for="<self-position>, <content-position>">left</dfn> <small>([[#self-alignment|self]], [[#content-distribution|content]])</small>
+		<dt><dfn for="justify-content, justify-self, justify-items">left</dfn> <small>(only 'justify-*')</small>
 		<dd>Aligns the <a>alignment subject</a> to be flush with
 			the <a>alignment container</a>’s <a href="https://www.w3.org/TR/css3-writing-modes/#line-left">line-left</a>
 			or <a>physical left</a> edge,
 			whichever is in the appropriate axis.
 			If the property's axis is not parallel with either left↔right axis,
 			this value behaves as ''start''.
 
-		<dt><dfn for="<self-position>, <content-position>">right</dfn> <small>([[#self-alignment|self]], [[#content-distribution|content]])</small>
+		<dt><dfn for="justify-content, justify-self, justify-items">right</dfn> <small>(only 'justify-*')</small>
 		<dd>Aligns the <a>alignment subject</a> to be flush with
 			the <a>alignment container</a>’s <a href="https://www.w3.org/TR/css3-writing-modes/#line-right">line-right</a>
 			or <a>physical right</a> edge,
@@ -361,10 +361,10 @@ Positional Alignment: the ''center'', ''start'', ''end'', ''self-start'', ''self
 			</pre>
 	</dl>
 
-	Note: ''left'' and ''right'' are excluded from <<self-position>>,
-	despite being valid [=positional alignment=] values for 'justify-self'/'justify-items',
-	because they are not allowed in 'align-self'.
-	They are instead explicitly included in the 'justify-self' and 'justify-items' grammars.
+	Note: ''left'' and ''right'' are excluded from <<self-position>> and <<content-position>>,
+	despite being valid [=positional alignment=] values for 'justify-content'/'justify-self'/'justify-items',
+	because they are not allowed in the 'align-*' properties.
+	They are instead explicitly included in the 'justify-*' properties’ grammars.
 
 	<aside class=example>
 		The ''<self-position>/start'' and ''<self-position>/end'' keywords