[css-inline-3] Add note about issue against adding first/last keywords to vertical-align.

Author: fantasai

css-inline-3/Overview.bs

@@ -156,6 +156,14 @@ Transverse Box Alignment: the 'vertical-align' property</h3>
 	<p class="advisement">
 		Authors should use this property ('vertical-align') instead of its longhands.
 
+	ISSUE: This property will gain ''first'' and ''last'' keywords,
+	like in the <a href="https://www.w3.org/TR/css-align-3/">box alignment properties</a>,
+	see [[css-align-3#baseline-values]].
+	The <a href="https://github.com/w3c/csswg-drafts/issues/861">open question</a>
+	is whether they should be added to 'alignment-baseline'
+	or a new <a>sub-property</a> should be created
+	to hold the ''first | last'' preference.
+
 <h4 id="alignment-baseline-property">
 Alignment Point: 'alignment-baseline' longhand</h4>