[css-align-3] Add note about when the axis for left/right doesn't match the element's left/right axis. Fixes w3c#2991.

Author: tabatkins

css-align-3/Overview.bs

@@ -327,16 +327,24 @@ Positional Alignment: the ''center'', ''start'', ''end'', ''self-start'', ''self
 			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''.
+			<span class=note>Currently, the only case where the property's axis
+			is not parallel with either left↔right axis
+			is in a column flexbox.</span>
 
 		<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,
 			whichever is in the appropriate axis.
+
 			If the property's axis is not parallel with either left↔right axis,
 			this value behaves as ''start''.
+			<span class=note>Currently, the only case where the property's axis
+			is not parallel with either left↔right axis
+			is in a column flexbox.</span>
 	</dl>
 
 	Two grammar terms are used to denote certain subsets of these values: