[css-display] Specify that 'run-in flow-root' blockifies to 'block flow', to be consistent with 'inline-block'. Fixes #1715.

Author: tabatkins

css-display-3/Overview.bs

@@ -635,6 +635,10 @@ Automatic Box Type Transformations</h3>
 		is <a>blockified</a>,
 		it becomes a ''block'' box
 		(losing its ''flow-root'' nature).
+		For consistency,
+		a ''run-in flow-root'' box
+		also [=blockifies=]
+		to a ''block'' box.
 
 	* If a [=layout-internal=] box is <a>blockified</a>,
 		its [=inner display type=] converts to ''flow''