[css-position-3] Clarify how the distributed/baseline/normal alignments are handled. w3c#6607

Author: tabatkins

css-position-3/Overview.bs

@@ -710,7 +710,9 @@ Absolute (and Fixed) Positioning</h3>
 	If only one [=inset property=] in a given axis is ''top/auto'',
 	it is set to zero.
 	If both [=inset properties=] in a given axis are ''top/auto'', then,
-	depending on the box’s [=self-alignment property=] in the relevant axis:
+	depending on the box’s [=self-alignment property=] in the relevant axis
+	(treating ''align-self/normal'' as ''align-self/start''
+	and any [=distributed alignment|distributed=], [=baseline alignment|baseline=], or ''align-self/stretch'' alignment value as its fallback alignment):
 
 	<dl class=switch>
 		<dt>for ''align-self/self-start'' alignment or its equivalent