[css-align-3] Add note calling out that justify-self:stretch is different from the content-alignment 'stretch' value.

Author: tabatkins

css-align-3/Overview.bs

@@ -762,6 +762,11 @@ Distributed Alignment: the ''<content-distribution>/stretch'', ''space-between''
 
 			The default [=fallback alignment=] for this value is ''flex-start''.
 			<span class='note'>(For layout modes other than flex layout, ''flex-start'' is identical to ''start''.)</span>
+
+			Note: This `align-content/stretch` definition applies to the [=content-distribution properties=];
+			the [=self-alignment properties=] 'justify-self'/'align-self' have their own ''justify-self/stretch'' value,
+			which can grow <em>or shrink</em> the [=alignment subject=]
+			to ensure it exactly fits the [=alignment container=].
 	</dl>
 
 	These values are represented with the <<content-distribution>> grammar term: