[css-align] Mark issue for safe vs. unsafe last-baseline alignment.

Author: fantasai

css-align/Overview.bs

@@ -305,6 +305,8 @@ Baseline Alignment: the ''baseline'' and ''last-baseline'' keywords</h3>
 			If the <a>alignment subject</a>’s position is not fully determined by <a>baseline alignment</a>,
 			the content is ''end''-aligned insofar as possible while preserving the baseline alignment.
 			(Content that has no <a>last baselines</a> is thus end-aligned.)
+
+			Issue: Should this ''end'' fallback be ''safe'' or ''unsafe''?
 	</dl>
 
 	These values give a box a <dfn export>baseline alignment preference</dfn>: