[css-align-3] Make fallback alignments properly correspond to baseline alignments in mixed writing-mode cases. w3c#3454

Author: tabatkins

css-align-3/Overview.bs

@@ -614,7 +614,9 @@ Baseline Alignment: the ''baseline'' keyword and ''first''/''last'' modifiers</h
 			of its [=baseline-sharing group=].
 			See [[#align-by-baseline]] for more details.
 
-			The <a>fallback alignment</a> for ''first baseline'' is ''start''.
+			The <a>fallback alignment</a> for ''first baseline'' is
+			''safe self-start'' (for [=self-alignment=])
+			or ''safe start'' (for [=content-distribution=]).
 
 		<dt><dfn lt="last baseline | last">last baseline</dfn>
 		<dd>
@@ -624,7 +626,9 @@ Baseline Alignment: the ''baseline'' keyword and ''first''/''last'' modifiers</h
 			of its [=baseline-sharing group=].̣
 			See [[#align-by-baseline]] for more details.
 
-			The <a>fallback alignment</a> for ''last baseline'' is ''safe end''.
+			The <a>fallback alignment</a> for ''last baseline'' is
+			''safe self-end'' (for [=self-alignment=])
+			or ''safe end'' (for [=content-distribution=]).
 	</dl>
 
 	When specified for 'align-content',